625 lines
24 KiB
Plaintext
625 lines
24 KiB
Plaintext
PEP: 3103
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Title: A Switch/Case Statement
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Version: $Revision$
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Last-Modified: $Date$
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Author: guido@python.org (Guido van Rossum)
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Status: Draft
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Type: Standards Track
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Python-Version: 3.0
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Content-Type: text/x-rst
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Created: 25-Jun-2006
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Post-History: 26-Jun-2006
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Abstract
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========
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Python-dev has recently seen a flurry of discussion on adding a switch
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statement. In this PEP I'm trying to extract my own preferences from
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the smorgasboard of proposals, discussing alternatives and explaining
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my choices where I can. I'll also indicate how strongly I feel about
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alternatives I discuss.
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This PEP should be seen as an alternative to PEP 275. My views are
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somewhat different from that PEP's author, but I'm grateful for the
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work done in that PEP.
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This PEP introduces canonical names for the many variants that have
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been discussed for different aspects of the syntax and semantics, such
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as "alternative 1", "school II", "option 3" and so on. Hopefully
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these names will help the discussion.
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Rationale
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=========
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A common programming idiom is to consider an expression and do
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different things depending on its value. This is usually done with a
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chain of if/elif tests; I'll refer to this form as the "if/elif
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chain". There are two main motivations to want to introduce new
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syntax for this idiom:
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- It is repetitive: the variable and the test operator, usually '=='
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or 'in', are repeated in each if/elif branch.
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- It is inefficient: when an expression matches the last test value
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(or no test value at all) it is compared to each of the preceding
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test values.
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Both of these complaints are relatively mild; there isn't a lot of
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readability or performance to be gained by writing this differently.
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Yet, some kind of switch statement is found in many languages and it
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is not unreasonable to expect that its addition to Python will allow
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us to write up certain code more cleanly and efficiently than before.
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There are forms of dispatch that are not suitable for the proposed
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switch statement; for example, when the number of cases is not
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statically known, or when it is desirable to place the code for
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different cases in different classes or files.
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Basic Syntax
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============
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I'm considering several variants of the syntax first proposed in PEP
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275 here. There are lots of other possibilities, but I don't see that
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they add anything.
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My current preference is alternative 2.
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I should note that all alternatives here have the "implicit break"
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property: at the end of the suite for a particular case, the control
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flow jumps to the end of the whole switch statement. There is no way
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to pass control from one case to another. This in contrast to C,
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where an explicit 'break' statement is required to prevent falling
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through to the next case.
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In all alternatives, the else-suite is optional. It is more Pythonic
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to use 'else' here rather than introducing a new reserved word,
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'default', as in C.
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Semantics are discussed in the next top-level section.
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Alternative 1
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-------------
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This is the preferred form in PEP 275::
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switch EXPR:
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case EXPR:
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SUITE
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case EXPR:
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SUITE
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...
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else:
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SUITE
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The main downside is that the suites where all the action is are
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indented two levels deep.
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Alternative 2
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-------------
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This is Fredrik Lundh's preferred form; it differs by not indenting
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the cases::
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switch EXPR:
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case EXPR:
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SUITE
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case EXPR:
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SUITE
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....
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else:
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SUITE
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Alternative 3
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-------------
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This is the same as alternative 2 but leaves out the colon after the
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switch::
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switch EXPR
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case EXPR:
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SUITE
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case EXPR:
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SUITE
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....
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else:
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SUITE
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The hope of this alternative is that it will not upset the auto-indent
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logic of the average Python-aware text editor less. But it looks
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strange to me.
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Alternative 4
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-------------
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This leaves out the 'case' keyword on the basis that it is redundant::
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switch EXPR:
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EXPR:
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SUITE
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EXPR:
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SUITE
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...
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else:
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SUITE
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Unfortunately now we are forced to indent the case expressions,
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because otherwise (at least in the absence of an 'else' keyword) the
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parser would have a hard time distinguishing between an unindented
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case expression (which continues the switch statement) or an unrelated
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statement that starts like an expression (such as an assignment or a
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procedure call). The parser is not smart enough to backtrack once it
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sees the colon. This is my least favorite alternative.
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Extended Syntax
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===============
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There is one additional concern that needs to be addressed
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syntactically. Often two or more values need to be treated the same.
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In C, this done by writing multiple case labels together without any
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code between them. The "fall through" semantics then mean that these
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are all handled by the same code. Since the Python switch will not
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have fall-through semantics (which have yet to find a champion) we
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need another solution. Here are some alternatives.
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Alternative A
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-------------
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Use::
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case EXPR:
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to match on a single expression; use::
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case EXPR, EXPR, ...:
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to match on mulltiple expressions. The is interpreted so that if EXPR
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is a parenthesized tuple or another expression whose value is a tuple,
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the switch expression must equal that tuple, not one of its elements.
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This means that we cannot use a variable to indicate multiple cases.
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While this is also true in C's switch statement, it is a relatively
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common occurrence in Python (see for example sre_compile.py).
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Alternative B
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-------------
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Use::
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case EXPR:
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to match on a single expression; use::
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case in EXPR_LIST:
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to match on multiple expressions. If EXPR_LIST is a single
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expression, the 'in' forces its interpretation as an iterable (or
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something supporting __contains__, in a minority semantics
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alternative). If it is multiple expressions, each of those is
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considered for a match.
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Alternative C
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-------------
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Use::
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case EXPR:
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to match on a single expression; use::
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case EXPR, EXPR, ...:
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to match on multiple expressions (as in alternative A); and use::
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case *EXPR:
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to match on the elements of an expression whose value is an iterable.
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The latter two cases can be combined, so that the true syntax is more
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like this::
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case [*]EXPR, [*]EXPR, ...:
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The `*` notation is similar to the use of prefix `*` already in use for
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variable-length parameter lists and for passing computed argument
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lists, and often proposed for value-unpacking (e.g. ``a, b, *c = X`` as
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an alternative to ``(a, b), c = X[:2], X[2:]``).
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Alternative D
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-------------
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This is a mixture of alternatives B and C; the syntax is like
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alternative B but instead of the 'in' keyword it uses '*'. This is
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more limited, but still allows the same flexibility. It uses::
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case EXPR:
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to match on a single expression and::
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case *EXPR:
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to match on the elements of an iterable. If one wants to specify
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multiple matches in one case, one can write this::
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case *(EXPR, EXPR, ...):
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or perhaps this (although it's a bit strange because the relative
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priority of '*' and ',' is different than elsewhere)::
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case * EXPR, EXPR, ...:
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Discussion
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----------
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Alternatives B, C and D are motivated by the desire to specify
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multiple cases with the same treatment using a variable representing a
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set (usually a tuple) rather than spelling them out. The motivation
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for this is usually that if one has several switches over the same set
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of cases it's a shame to have to spell out all the alternatives each
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time. An additional motivation is to be able to specify *ranges* to
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be matched easily and efficiently, similar to Pascal's "1..1000:"
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notation. At the same time we want to prevent the kind of mistake
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that is common in exception handling (and which will be addressed in
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Python 3000 by changing the syntax of the except clause): writing
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"case 1, 2:" where "case (1, 2):" was meant, or vice versa.
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The case could be made that the need is insufficient for the added
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complexity; C doesn't have a way to express ranges either, and it's
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used a lot more than Pascal these days. Also, if a dispatch method
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based on dict lookup is chosen as the semantics, large ranges could be
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inefficient (consider range(1, sys.maxint)).
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All in all my preferences are (from most to least favorite) B, A, D',
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C, where D' is D without the third possibility.
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Semantics
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=========
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There are several issues to review before we can choose the right
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semantics.
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If/Elif Chain vs. Dict-based Dispatch
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-------------------------------------
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There are several main schools of thought about the switch statement's
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semantics:
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- School I wants to define the switch statement in term of an
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equivalent if/elif chain (possibly with some optimization thrown
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in).
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- School II prefers to think of it as a dispatch on a precomputed
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dict. There are different choices for when the precomputation
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happens.
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- There's also school III, which agrees with school I that the
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definition of a switch statement should be in terms of an equivalent
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if/elif chain, but concedes to the optimization camp that all
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expressions involved must be hashable.
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We need to further separate school I into school Ia and school Ib:
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- School Ia has a simple position: a switch statement is translated to
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an equivalent if/elif chain, and that's that. It should not be
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linked to optimization at all. That is also my main objection
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against this school: without any hint of optimization, the switch
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statement isn't attractive enough to warrant new syntax.
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- School Ib has a more complex position: it agrees with school II that
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optimization is important, and is willing to concede the compiler
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certain liberties to allow this. (For example, PEP 275 Solution 1.)
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In particular, hash() of the switch and case expressions may or may
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not be called (so it should be side-effect-free); and the case
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expressions may not be evaluated each time as expected by the
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if/elif chain behavior, so the case expressions should also be
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side-effect free. My objection to this (elaborated below) is that
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if either the hash() or the case expressions aren't
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side-effect-free, optimized and unoptimized code may behave
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differently.
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School II grew out of the realization that optimization of commonly
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found cases isn't so easy, and that it's better to face this head on.
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This will become clear below.
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The differences between school I (mostly school Ib) and school II are
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threefold:
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- When optimizing using a dispatch dict, if either the switch
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expression or the case expressions are unhashable (in which case
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hash() raises an exception), school Ib requires catching the hash()
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failure and falling back to an if/elif chain. School II simply lets
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the exception happen. The problem with catching an exception in
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hash() as required by school Ib, is that this may hide a genuine
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bug. A possible way out is to only use a dispatch dict if all case
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expressions are ints, strings or other built-ins with known good
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hash behavior, and to only attempt to hash the switch expression if
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it is also one of those types. Type objects should probably also be
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supported here. This is the (only) problem that school III
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addresses.
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- When optimizing using a dispatch dict, if the hash() function of any
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expression involved returns an incorrect value, under school Ib,
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optimized code will not behave the same as unoptimized code. This
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is a well-known problem with optimization-related bugs, and waste
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lots of developer time. Under school II, in this situation
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incorrect results are produced at least consistently, which should
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make debugging a bit easier. The way out proposed for the previous
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bullet would also help here.
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- School Ib doesn't have a good optimization strategy if the case
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expressions are named constants. The compiler cannot know their
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values for sure, and it cannot know whether they are truly constant.
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As a way out, it has been proposed to re-evaluate the expression
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corresponding to the case once the dict has identified which case
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should be taken, to verify that the value of the expression didn't
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change. But strictly speaking, all the case expressions occurring
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before that case would also have to be checked, in order to preserve
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the true if/elif chain semantics, thereby completely killing the
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optimization. Another proposed solution is to have callbacks
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notifying the dispatch dict of changes in the value of variables or
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attributes involved in the case expressions. But this is not likely
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implementable in the general case, and would require many namespaces
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to bear the burden of supporting such callbacks, which currently
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don't exist at all.
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- Finally, there's a difference of opinion regarding the treatment of
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duplicate cases (i.e. two or more cases with match expressions that
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evaluates to the same value). School I wants to treat this the same
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is an if/elif chain would treat it (i.e. the first match wins and
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the code for the second match is silently unreachable); school II
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wants this to be an error at the time the dispatch dict is frozen
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(so dead code doesn't go undiagnosed).
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School I sees trouble in school II's approach of pre-freezing a
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dispatch dict because it places a new and unusual burden on
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programmers to understand exactly what kinds of case values are
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allowed to be frozen and when the case values will be frozen, or they
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might be surprised by the switch statement's behavior.
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School II doesn't believe that school Ia's unoptimized switch is worth
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the effort, and it sees trouble in school Ib's proposal for
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optimization, which can cause optimized and unoptimized code to behave
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differently.
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In addition, school II sees little value in allowing cases involving
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unhashable values; after all if the user expects such values, they can
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just as easily write an if/elif chain. School II also doesn't believe
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that it's right to allow dead code due to overlapping cases to occur
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unflagged, when the dict-based dispatch implementation makes it so
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easy to trap this.
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However, there are some use cases for overlapping/duplicate cases.
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Suppose you're switching on some OS-specific constants (e.g. exported
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by the os module or some module like that). You have a case for each.
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But on some OS, two different constants have the same value (since on
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that OS they are implemented the same way -- like O_TEXT and O_BINARY
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on Unix). If duplicate cases are flagged as errors, your switch
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wouldn't work at all on that OS. It would be much better if you could
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arrange the cases so that one case has preference over another.
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There's also the (more likely) use case where you have a set of cases
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to be treated the same, but one member of the set must be treated
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differently. It would be convenient to put the exception in an
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earlier case and be done with it.
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(Yes, it seems a shame not to be able to diagnose dead code due to
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accidental case duplication. Maybe that's less important, and
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pychecker can deal with it? After all we don't diagnose duplicate
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method definitions either.)
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This suggests school IIb: like school II but redundant cases must be
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resolved by choosing the first match. This is trivial to implement
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when building the dispatch dict (skip keys already present).
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(An alternative would be to introduce new syntax to indicate "okay to
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have overlapping cases" or "ok if this case is dead code" but I find
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that overkill.)
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Personally, I'm in school II: I believe that the dict-based dispatch
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is the one true implementation for switch statements and that we
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should face the limitiations up front, so that we can reap maximal
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benefits. I'm leaning towards school IIb -- duplicate cases should be
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resolved by the ordering of the cases instead of flagged as errors.
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When to Freeze the Dispatch Dict
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--------------------------------
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For the supporters of school II (dict-based dispatch), the next big
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dividing issue is when to create the dict used for switching. I call
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this "freezing the dict".
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The main problem that makes this interesting is the observation that
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Python doesn't have named compile-time constants. What is
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conceptually a constant, such as re.IGNORECASE, is a variable to the
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compiler, and there's nothing to stop crooked code from modifying its
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value.
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Option 1
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''''''''
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The most limiting option is to freeze the dict in the compiler. This
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would require that the case expressions are all literals or
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compile-time expressions involving only literals and operators whose
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semantics are known to the compiler, since with the current state of
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Python's dynamic semantics and single-module compilation, there is no
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hope for the compiler to know with sufficient certainty the values of
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any variables occurring in such expressions. This is widely though
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not universally considered too restrictive.
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Raymond Hettinger is the main advocate of this approach. He proposes
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a syntax where only a single literal of certain types is allowed as
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the case expression. It has the advantage of being unambiguous and
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easy to implement.
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My main complaint about this is that by disallowing "named constants"
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we force programmers to give up good habits. Named constants are
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introduced in most languages to solve the problem of "magic numbers"
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occurring in the source code. For example, sys.maxint is a lot more
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readable than 2147483647. Raymond proposes to use string literals
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instead of named "enums", observing that the string literal's content
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can be the name that the constant would otherwise have. Thus, we
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could write "case 'IGNORECASE':" instead of "case re.IGNORECASE:".
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However, if there is a spelling error in the string literal, the case
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will silently be ignored, and who knows when the bug is detected. If
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there is a spelling error in a NAME, however, the error will be caught
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as soon as it is evaluated. Also, sometimes the constants are
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externally defined (e.g. when parsing a file format like JPEG) and we
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can't easily choose appropriate string values. Using an explicit
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mappping dict sounds like a poor hack.
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Option 2
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''''''''
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The oldest proposal to deal with this is to freeze the dispatch dict
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the first time the switch is executed. At this point we can assume
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that all the named "constants" (constant in the programmer's mind,
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though not to the compiler) used as case expressions are defined --
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otherwise an if/elif chain would have little chance of success either.
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Assuming the switch will be executed many times, doing some extra work
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the first time pays back quickly by very quick dispatch times later.
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An objection to this option is that there is no obvious object where
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the dispatch dict can be stored. It can't be stored on the code
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object, which is supposed to be immutable; it can't be stored on the
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function object, since many function objects may be created for the
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same function (e.g. for nested functions). In practice, I'm sure that
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something can be found; it could be stored in a section of the code
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object that's not considered when comparing two code objects or when
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pickling or marshalling a code object; or all switches could be stored
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in a dict indexed by weak references to code objects. The solution
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should also be careful not to leak switch dicts between multiple
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interpreters.
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Another objection is that the first-use rule allows obfuscated code
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like this::
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def foo(x, y):
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switch x:
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case y:
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print 42
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To the untrained eye (not familiar with Python) this code would be
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equivalent to this::
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def foo(x, y):
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if x == y:
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print 42
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but that's not what it does (unless it is always called with the same
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value as the second argument). This has been addressed by suggesting
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that the case expressions should not be allowed to reference local
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variables, but this is somewhat arbitrary.
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A final objection is that in a multi-threaded application, the
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first-use rule requires intricate locking in order to guarantee the
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correct semantics. (The first-use rule suggests a promise that side
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effects of case expressions are incurred exactly once.) This may be
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as tricky as the import lock has proved to be, since the lock has to
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be held while all the case expressions are being evaluated.
|
||
|
||
Option 3
|
||
''''''''
|
||
|
||
A proposal that has been winning support (including mine) is to freeze
|
||
a switch's dict when the innermost function containing it is defined.
|
||
The switch dict is stored on the function object, just as parameter
|
||
defaults are, and in fact the case expressions are evaluated at the
|
||
same time and in the same scope as the parameter defaults (i.e. in the
|
||
scope containing the function definition).
|
||
|
||
This option has the advantage of avoiding many of the finesses needed
|
||
to make option 2 work: there's no need for locking, no worry about
|
||
immutable code objects or multiple interpreters. It also provides a
|
||
clear explanation for why locals can't be referenced in case
|
||
expressions.
|
||
|
||
This option works just as well for situations where one would
|
||
typically use a switch; case expressions involving imported or global
|
||
named constants work exactly the same way as in option 2, as long as
|
||
they are imported or defined before the function definition is
|
||
encountered.
|
||
|
||
A downside however is that the dispatch dict for a switch inside a
|
||
nested function must be recomputed each time the nested function is
|
||
defined. For certain "functional" styles of programming this may make
|
||
switch unattractive in nested functions. (Unless all case expressions
|
||
are compile-time constants; then the compiler is of course free to
|
||
optimize away the swich freezing code and make the dispatch table part
|
||
of the code object.)
|
||
|
||
Another downside is that under this option, there's no clear moment
|
||
when the dispatch dict is frozen for a switch that doesn't occur
|
||
inside a function. There are a few pragmatic choices for how to treat
|
||
a switch outside a function:
|
||
|
||
(a) Disallow it.
|
||
(b) Translate it into an if/elif chain.
|
||
(c) Allow only compile-time constant expressions.
|
||
(d) Compute the dispatch dict each time the switch is reached.
|
||
(e) Like (b) but tests that all expressions evaluated are hashable.
|
||
|
||
Of these, (a) seems too restrictive: it's uniformly worse than (c);
|
||
and (d) has poor performance for little or no benefits compared to
|
||
(b). It doesn't make sense to have a performance-critical inner loop
|
||
at the module level, as all local variable references are slow there;
|
||
hence (b) is my (weak) favorite. Perhaps I should favor (e), which
|
||
attempts to prevent atypical use of a switch; examples that work
|
||
interactively but not in a function are annoying. In the end I don't
|
||
think this issue is all that important (except it must be resolved
|
||
somehow) and am willing to leave it up to whoever ends up implementing
|
||
it.
|
||
|
||
When a switch occurs in a class but not in a function, we can freeze
|
||
the dispatch dict at the same time the temporary function object
|
||
representing the class body is created. This means the case
|
||
expressions can reference module globals but not class variables.
|
||
Alternatively, if we choose (b) above, we could choose this
|
||
implementation inside a class definition as well.
|
||
|
||
Option 4
|
||
''''''''
|
||
|
||
There are a number of proposals to add a construct to the language
|
||
that makes the concept of a value pre-computed at function definition
|
||
time generally available, without tying it either to parameter default
|
||
values or case expressions. Some keywords proposed include 'const',
|
||
'static', 'only' or 'cached'. The associated syntax and semantics
|
||
vary.
|
||
|
||
These proposals are out of scope for this PEP, except to suggest that
|
||
*if* such a proposal is accepted, there are two ways for the switch to
|
||
benefit: we could require case expressions to be either compile-time
|
||
constants or pre-computed values; or we could make pre-computed values
|
||
the default (and only) evaluation mode for case expressions. The
|
||
latter would be my preference, since I don't see a use for more
|
||
dynamic case expressions that isn't addressed adequately by writing an
|
||
explicit if/elif chain.
|
||
|
||
|
||
Conclusion
|
||
==========
|
||
|
||
It is too early to decide. I'd like to see at least one completed
|
||
proposal for pre-computed values before deciding. In the mean time,
|
||
Python is fine without a switch statement, and perhaps those who claim
|
||
it would be a mistake to add one are right.
|
||
|
||
|
||
Copyright
|
||
=========
|
||
|
||
This document has been placed in the public domain.
|
||
|
||
|
||
|
||
..
|
||
Local Variables:
|
||
mode: indented-text
|
||
indent-tabs-mode: nil
|
||
sentence-end-double-space: t
|
||
fill-column: 70
|
||
coding: utf-8
|
||
End:
|