python-peps/pep-0308.txt

PEP: 308
Title: If-then-else expression
Version: $Revision$
Last-Modified: $Date$
Author: Guido van Rossum
Status: Active
Type: Standards Track
Content-Type: text/plain
Created: 7-Feb-2003
Post-History: 7-Feb-2003


Introduction

    Requests for an if-then-else ("ternary") expression keep coming up
    on comp.lang.python.  This PEP contains a concrete proposal of a
    fairly Pythonic syntax.  This is the community's one chance: if
    this PEP is approved with a clear majority, it will be implemented
    in Python 2.4.  If not, the PEP will be augmented with a summary
    of the reasons for rejection and the subject better not come up
    again.  While I am the author of this PEP, I am neither in favor
    nor against this proposal; it is up to the community to decide.
    If the community can't decide, I'll reject the PEP.


Proposal

    The proposed syntax is as follows:

        <expression1> if <condition> else <expression2>

    This is evaluated like this:

    - First, <condition> is evaluated.

    - If <condition> is true, <expression1> is evaluated and is the
      result of the whole thing.

    - If <condition> is false, <expression2> is evaluated and is the
      result of the whole thing.

    Note that at most one of <expression1> and <expression2> is
    evaluated.  This is called a "shortcut expression"; it is similar
    to the way the second operand of 'and' / 'or' is only evaluated if
    the first operand is true / false.

    To disambiguate this in the context of other operators, the
    "if...else" part in the middle acts like a left-associative binary
    operator with a priority lower than that of "or", and higher than
    that of "lambda".

    Examples of how this works out:

        x if C else y if D else z <==> x if C else (y if D else z)
        x or y if C else z        <==> (x or y) if C else z
	x if C else y or z        <==> x if C else (y or z)
        lambda: x if C else y     <==> lambda: (x if C else y)
        x if C else lambda: y     <==> SyntaxError
        x if C else y, z          <==> (x if C else y), z
        x, y if C else z          <==> x, (y if C else z)

    Note: a common way to emulate an if-then-else expression is:

        <condition> and <expression1> or <expression2>

    However, this doesn't work the same way: it returns <expression2>
    when <expression1> is false!  See FAQ 4.16 for alternatives that
    work -- however, they are pretty ugly and require much more
    effort to understand.


Alternatives

    Many C-derived languages use this syntax:

        <condition> ? <expression1> : <expression2>

    Eric Raymond even implemented this.  I reject this for several
    reasons: the colon already has many uses in Python (even though it
    would actually not be ambiguous, because the question mark
    requires a matching colon); for people not used to C-derived
    language, it is hard to understand.

    ---

    David Ascher proposed a variant that doesn't have this problem:

        <condition> ? <expression1> ! <expression2>

    While cute, this suffers from the Perlish problem of using
    arbitrary punctuation with an arbitrary meaning; and it's no
    easier to understand than the ?: form.

    ---

    If we could live with adding a new keyword, we could use:

        if <condition> then <expression1> else <expression2>

    Apart from the problem of introducing a new keyword for a minor
    feature, this also suffers from ambiguity at the start of a
    statement; for example:

        if verbose then sys.stdout.write("hello\n") else None

    could be an syntactically correct expression statement, but starts
    with 'if', which makes the parser believe it is the start of an
    'if' statement.  To resolve this, the syntax would have to require
    parentheses, which makes it uglier.  However, this form has the
    advantage of evaluating strictly from left to right (not that that
    is a requirement for being Pythonic -- list comprehensions don't).

    ---

    To deal with the problem of adding a new keyword, this variant has
    been proposed:

        if <condition> : <expression1> else <expression2>

    This has the same ambiguity problem as the previous one (I would
    even say more so), and lacks symmetry.  It also begs the question
    why there isn't a colon after the 'else'.  But this:

        if <condition> : <expression1> else: <expression2>

    is even more confusing because it resembles the if statement so
    much.  (A solution that *doesn't* resemble the if statement is
    better IMO since it should be obvious at first glance whether
    we're dealing with an if expression or with an if statement.
    Placing the 'if' in the middle somehow satisfies this
    requirement.)

    ---

    Many people suggest adding a new builtin instead of extending the
    syntax of the language, e.g.:

        ifelse(condition, expression1, expression2)

    This won't work the way a syntax extension will because both
    expression1 and expression2 must be evaluated before the function
    is called.  There's no way to short-circuit the expression
    evaluation.


Variations

    It has been proposed to make the 'else' part optional.  This would
    be a really bad idea.  I showed:

        x = e if C

    to several people.  They all thought that if C was false, it would
    leave x unchanged.  So don't even think about this one!


Copyright

    This document has been placed in the public domain.



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