PEP: 340 Title: Anonymous Block Statements Version: $Revision$ Last-Modified: $Date$ Author: Guido van Rossum Status: Draft Type: Standards Track Content-Type: text/plain Created: 27-Apr-2005 Post-History: Introduction This PEP proposes a new type of compound statement which can be used for resource management purposes, and a new iterator API to go with it. The new statement type is provisionally called the block-statement because the keyword to be used has not yet been chosen. This PEP competes with several other PEPs: PEP 288 (Generators Attributes and Exceptions; only the second part), PEP 310 (Reliable Acquisition/Release Pairs), and PEP 325 (Resource-Release Support for Generators). This proposal is just a strawman; we've had a heated debate about this on python-dev recently [1], and I figured it would be time to write up a precise spec in PEP form. Motivation and Use Cases TBD. Specification: the Iteration Exception Hierarchy Two new built-in exceptions are defined, and StopIteration is moved in the exception hierarchy: class Iteration(Exception): pass class StopIteration(Iteration): pass class ContinueIteration(Iteration): def __init__(self, value=None): self.value = None Specification: the __next__() Method A new method for iterators is proposed, called __next__(). It takes one optional argument, which defaults to None. If not None, the argument must be an Iteration instance. Calling the __next__() method without argument or with None is equivalent to using the old iterator API, next(). For backwards compatibility, it is recommended that iterators also implement a next() method as an alias for calling the __next__() method without an argument. Calling the __next__() method with a StopIteration instance signals the iterator that the caller wants to abort the iteration sequence; the iterator should respond by doing any necessary cleanup and raising StopIteration. Calling it with a ContinueIteration instance signals the iterator that the caller wants to continue the iteration; the ContinueIteration exception has a 'value' attribute which may be used by the iterator as a hint on what to do next. Calling it with a (base class) Iteration instance is the same as calling it with None. Specification: the next() Built-in Function This is a built-in function defined as follows: def next(itr, arg=None): nxt = getattr(itr, "__next__", None) if nxt is not None: return nxt(arg) if arg is None: return itr.next() raise TypeError("next() with arg for old-style iterator") Specification: the 'for' Loop A small change in the translation of the for-loop is proposed. The statement for VAR1 in EXPR1: BLOCK1 else: BLOCK2 will be translated as follows: itr = iter(EXPR1) arg = None while True: try: VAR1 = next(itr, arg) except StopIteration: break arg = None BLOCK1 else: BLOCK2 (However, 'it' and 'arg' are hidden from the user, their scope ends when the while-loop is exited, and they are not shared with nested or outer for-loops, and the user cannot override the built-ins referenced.) Specification: the Extended 'continue' Statement In the translation of the for-loop, inside BLOCK1, the new syntax continue EXPR2 is legal and is translated into arg = ContinueIteration(EXPR2) continue (Where 'arg' references the corresponding hidden variable from the previous section.) This is also the case in the body of the block-statement proposed below. Specification: the Anonymous Block Statement A new statement is proposed with the syntax block EXPR1 as VAR1: BLOCK1 Here, 'block' and 'as' are new keywords; EXPR1 is an arbitrary expression (but not an expression-list) and VAR1 is an arbitrary assignment target (which may be a comma-separated list). The "as VAR1" part is optional; if omitted, the assignment to VAR1 in the translation below is omitted (but the next() call is not!). The choice of the 'block' keyword is contentious; it has even been proposed not to use a keyword at all. PEP 310 uses 'with' for similar semantics, but I would like to reserve that for a with-statement similar to the one found in Pascal and VB. To sidestep this issue momentarily I'm using 'block' until we can agree on a keyword. (I just found that the C# designers don't like 'with' [2].) Note that it is left in the middle whether a block-statement represents a loop or not; this is up to the iterator, but in the most common case BLOCK1 is executed exactly once. The translation is subtly different from the translation of a for-loop: iter() is not called, so EXPR1 should already be an iterator (not just an iterable); and the iterator is guaranteed to be exhausted when the block-statement is left: itr = EXPR1 val = arg = None ret = False while True: try: VAR1 = next(itr, arg) except StopIteration: if ret: return val if val is not None: raise val break try: val = arg = None BLOCK1 except Exception, val: arg = StopIteration() (Again, 'itr' etc. are hidden, and the user cannot override the built-ins.) The "raise val" translation is inexact; this is supposed to re-raise the exact exception that was raised inside BLOCK1, with the same traceback. We can't use a bare raise-statement because we've just caught StopIteration. Inside BLOCK1, the following special translations apply: - "continue" and "continue EXPR2" are always legal; the latter is translated as shown earlier: arg = ContinueIteration(EXPR2) continue - "break" is always legal; it is translated into: arg = StopIteration() continue - "return EXPR3" is only legal when the block-statement is contained in a function definition; it is translated into: val = EXPR3 ret = True arg = StopIteration() continue The net effect is that break, continue and return behave much the same as if the block-statement were a for-loop, except that the iterator gets a chance at resource cleanup before the block-statement is left. The iterator also gets a chance if the block-statement is left through raising an exception. Specification: Generator Exception Handling Generators will implement the new __next__() method API, as well as the old argument-less next() method. Generators will be allowed to have a yield statement inside a try-finally statement. The expression argument to the yield-statement will become optional (defaulting to None). The yield-statement will be allowed to be used on the right-hand side of an assignment; in that case it is referred to as yield-expression. The value of this yield-expression is None unless __next__() was called with a ContinueIteration argument; see below. A yield-expression must always be parenthesized except when it occurs at the top-level expression on the right-hand side of an assignment. So x = yield 42 x = yield x = 12 + (yield 42) x = 12 + (yield) foo(yield 42) foo(yield) are all legal, but x = 12 + yield 42 x = 12 + yield foo(yield 42, 12) foo(yield, 12) are all illegal. (Some of the edge cases are motivated by the current legality of "yield 12, 42".) When __next__() is called with a StopIteration instance argument, the yield statement that is resumed by the __next__() call will raise this StopIteration exception. The generator should re-raise this exception; it should not yield another value. When the *initial* call to __next__() receives a StopIteration instance argument, the generator's execution is aborted and the exception is re-raised without passing control to the generator's body. When __next__() is called with a ContinueIteration instance argument, the yield-expression that it resumes will return the value attribute of the argument. If it resumes a yield-statement, the value is ignored. When the *initial* call to __next__() receives a ContinueIteration instance argument, the generator's execution is started normally; the argument's value attribute is ignored. When a generator that has not yet terminated is garbage-collected (either through reference counting or by the cyclical garbage collector), its __next__() method is called once with a StopIteration instance argument. Together with the requirement that __next__() should always re-raise a StopIteration argument, this guarantees the eventual activation of any finally-clauses that were active when the generator was last suspended. Of course, under certain circumstances the generator may never be garbage-collected. This is no different than the guarantees that are made about finalizers (__del__() methods) of other objects. Note: the syntactic extensions to yield make it use very similar to that in Ruby. This is intentional. Do note that in Python the block passes a value to the generator using "continue EXPR" rather than "return EXPR", and the underlying mechanism whereby control is passed between the generator and the block is completely different. Blocks in Python are not compiled into thunks; rather, yield suspends execution of the generator's frame. Some edge cases work differently; in Python, you cannot save the block for later use, and you cannot test whether there is a block or not. Specification: Alternative __next__() and Generator Exception Handling The above specification doesn't let the generator handle general exceptions. If we want that, we could modify the __next__() API to take either a value or an exception argument. When it is an Exception instance, it is raised at the point of the resuming yield; otherwise it is returned from the yield-expression (or ignored by a yield-statement). Wrapping a regular value in a ContinueIteration is then no longer necessary. The translation of a block-statement would become: itr = EXPR1 arg = val = None ret = False while True: try: VAR1 = next(itr, arg) except StopIteration: if ret: return val break try: arg = val = None ret = False BLOCK1 except Exception, arg: pass The translation of "continue EXPR2" would become: arg = EXPR2 continue The translation of "break" inside a block-statement would become: arg = StopIteration() continue The translation of "return EXPR3" inside a block-statement would become: val = EXPR3 arg = StopIteration() ret = True continue The translation of a for-loop would be the same as indicated earlier (inside a for-loop only the translation of "continue EXPR2" is changed; break and return translate to themselves in that case). Alternatives Considered TBD. Examples TBD. Acknowledgements In no useful order: Alex Martelli, Barry Warsaw, Bob Ippolito, Brett Cannon, Brian Sabbey, Doug Landauer, Fredrik Lundh, Greg Ewing, Holger Krekel, Jason Diamond, Jim Jewett, Josiah Carlson, Ka-Ping Yee, Michael Chermside, Michael Hudson, Nick Coghlan, Paul Moore, Phillip Eby, Raymond Hettinger, Samuele Pedroni, Shannon Behrens, Steven Bethard, Terry Reedy, Tim Delaney, Aahz, and others. Thanks all for a valuable discussion and ideas. References [1] http://mail.python.org/pipermail/python-dev/2005-April/052821.html [2] http://msdn.microsoft.com/vcsharp/programming/language/ask/withstatement/ Copyright This document has been placed in the public domain.