PEP 255 – Simple Generators
- Neil Schemenauer <nas at arctrix.com>, Tim Peters <tim.peters at gmail.com>, Magnus Lie Hetland <magnus at hetland.org>
- Standards Track
- 14-Jun-2001, 23-Jun-2001
Table of Contents
- Specification: Yield
- Specification: Return
- Specification: Generators and Exception Propagation
- Specification: Try/Except/Finally
- Q & A
- Why not a new keyword instead of reusing
- Why a new keyword for
yield? Why not a builtin function instead?
- Then why not some other special syntax without a new keyword?
- Why allow
returnat all? Why not force termination to be spelled
- Then why not allow an expression on
- Why not a new keyword instead of reusing
- BDFL Pronouncements
- Reference Implementation
- Footnotes and References
This PEP introduces the concept of generators to Python, as well as a new
statement used in conjunction with them, the
When a producer function has a hard enough job that it requires maintaining state between values produced, most programming languages offer no pleasant and efficient solution beyond adding a callback function to the producer’s argument list, to be called with each value produced.
tokenize.py in the standard library takes this approach: the
caller must pass a tokeneater function to
tokenize(), called whenever
tokenize() finds the next token. This allows tokenize to be coded in a
natural way, but programs calling tokenize are typically convoluted by the need
to remember between callbacks which token(s) were seen last. The tokeneater
tabnanny.py is a good example of that, maintaining a state
machine in global variables, to remember across callbacks what it has already
seen and what it hopes to see next. This was difficult to get working
correctly, and is still difficult for people to understand. Unfortunately,
that’s typical of this approach.
An alternative would have been for tokenize to produce an entire parse of the Python program at once, in a large list. Then tokenize clients could be written in a natural way, using local variables and local control flow (such as loops and nested if statements) to keep track of their state. But this isn’t practical: programs can be very large, so no a priori bound can be placed on the memory needed to materialize the whole parse; and some tokenize clients only want to see whether something specific appears early in the program (e.g., a future statement, or, as is done in IDLE, just the first indented statement), and then parsing the whole program first is a severe waste of time.
Another alternative would be to make tokenize an iterator,
next token whenever its
.next() method is invoked. This is pleasant for the
caller in the same way a large list of results would be, but without the memory
and “what if I want to get out early?” drawbacks. However, this shifts the
burden on tokenize to remember its state between
.next() invocations, and
the reader need only glance at
tokenize.tokenize_loop() to realize what a
horrid chore that would be. Or picture a recursive algorithm for producing the
nodes of a general tree structure: to cast that into an iterator framework
requires removing the recursion manually and maintaining the state of the
traversal by hand.
A fourth option is to run the producer and consumer in separate threads. This allows both to maintain their states in natural ways, and so is pleasant for both. Indeed, Demo/threads/Generator.py in the Python source distribution provides a usable synchronized-communication class for doing that in a general way. This doesn’t work on platforms without threads, though, and is very slow on platforms that do (compared to what is achievable without threads).
A final option is to use the Stackless  (PEP 219) variant implementation of Python instead, which supports lightweight coroutines. This has much the same programmatic benefits as the thread option, but is much more efficient. However, Stackless is a controversial rethinking of the Python core, and it may not be possible for Jython to implement the same semantics. This PEP isn’t the place to debate that, so suffice it to say here that generators provide a useful subset of Stackless functionality in a way that fits easily into the current CPython implementation, and is believed to be relatively straightforward for other Python implementations.
That exhausts the current alternatives. Some other high-level languages provide pleasant solutions, notably iterators in Sather , which were inspired by iterators in CLU; and generators in Icon , a novel language where every expression is a generator. There are differences among these, but the basic idea is the same: provide a kind of function that can return an intermediate result (“the next value”) to its caller, but maintaining the function’s local state so that the function can be resumed again right where it left off. A very simple example:
def fib(): a, b = 0, 1 while 1: yield b a, b = b, a+b
fib() is first invoked, it sets a to 0 and b to 1, then yields b
back to its caller. The caller sees 1. When
fib is resumed, from its
point of view the
yield statement is really the same as, say, a
fib continues after the yield with all local state intact. a
and b then become 1 and 1, and
fib loops back to the
1 to its invoker. And so on. From
fib’s point of view it’s just
delivering a sequence of results, as if via callback. But from its caller’s
point of view, the
fib invocation is an iterable object that can be resumed
at will. As in the thread approach, this allows both sides to be coded in the
most natural ways; but unlike the thread approach, this can be done efficiently
and on all platforms. Indeed, resuming a generator should be no more expensive
than a function call.
The same kind of approach applies to many producer/consumer functions. For
tokenize.py could yield the next token instead of invoking a
callback function with it as argument, and tokenize clients could iterate over
the tokens in a natural way: a Python generator is a kind of Python
iterator, but of an especially powerful kind.
A new statement is introduced:
yield_stmt: "yield" expression_list
yield is a new keyword, so a
future statement (PEP 236) is needed to phase
this in: in the initial release, a module desiring to use generators must
include the line:
from __future__ import generators
near the top (see PEP 236) for details). Modules using the identifier
yield without a
future statement will trigger warnings. In the
yield will be a language keyword and the
statement will no longer be needed.
yield statement may only be used inside functions. A function that
yield statement is called a generator function. A generator
function is an ordinary function object in all respects, but has the new
CO_GENERATOR flag set in the code object’s co_flags member.
When a generator function is called, the actual arguments are bound to function-local formal argument names in the usual way, but no code in the body of the function is executed. Instead a generator-iterator object is returned; this conforms to the iterator protocol, so in particular can be used in for-loops in a natural way. Note that when the intent is clear from context, the unqualified name “generator” may be used to refer either to a generator-function or a generator-iterator.
Each time the
.next() method of a generator-iterator is invoked, the code
in the body of the generator-function is executed until a
return statement (see below) is encountered, or until the end of the body
yield statement is encountered, the state of the function is frozen,
and the value of expression_list is returned to
.next()’s caller. By
“frozen” we mean that all local state is retained, including the current
bindings of local variables, the instruction pointer, and the internal
evaluation stack: enough information is saved so that the next time
.next() is invoked, the function can proceed exactly as if the
statement were just another external call.
yield statement is not allowed in the
try clause of a
try/finally construct. The difficulty is that there’s no guarantee the
generator will ever be resumed, hence no guarantee that the finally block will
ever get executed; that’s too much a violation of finally’s purpose to bear.
Restriction: A generator cannot be resumed while it is actively running:
>>> def g(): ... i = me.next() ... yield i >>> me = g() >>> me.next() Traceback (most recent call last): ... File "<string>", line 2, in g ValueError: generator already executing
A generator function can also contain return statements of the form:
Note that an expression_list is not allowed on return statements in the body of a generator (although, of course, they may appear in the bodies of non-generator functions nested within the generator).
When a return statement is encountered, control proceeds as in any function
return, executing the appropriate
finally clauses (if any exist). Then a
StopIteration exception is raised, signalling that the iterator is
StopIteration exception is also raised if control flows off
the end of the generator without an explicit return.
Note that return means “I’m done, and have nothing interesting to return”, for both generator functions and non-generator functions.
Note that return isn’t always equivalent to raising
difference lies in how enclosing
try/except constructs are treated. For
>>> def f1(): ... try: ... return ... except: ... yield 1 >>> print list(f1()) 
because, as in any function,
return simply exits, but:
>>> def f2(): ... try: ... raise StopIteration ... except: ... yield 42 >>> print list(f2()) 
StopIteration is captured by a bare
except, as is any
Specification: Generators and Exception Propagation
If an unhandled exception– including, but not limited to,
–is raised by, or passes through, a generator function, then the exception is
passed on to the caller in the usual way, and subsequent attempts to resume the
generator function raise
StopIteration. In other words, an unhandled
exception terminates a generator’s useful life.
Example (not idiomatic but to illustrate the point):
>>> def f(): ... return 1/0 >>> def g(): ... yield f() # the zero division exception propagates ... yield 42 # and we'll never get here >>> k = g() >>> k.next() Traceback (most recent call last): File "<stdin>", line 1, in ? File "<stdin>", line 2, in g File "<stdin>", line 2, in f ZeroDivisionError: integer division or modulo by zero >>> k.next() # and the generator cannot be resumed Traceback (most recent call last): File "<stdin>", line 1, in ? StopIteration >>>
As noted earlier,
yield is not allowed in the
try clause of a
try/finally construct. A consequence is that generators should allocate
critical resources with great care. There is no restriction on
otherwise appearing in
except clauses, or in the
try clause of a
>>> def f(): ... try: ... yield 1 ... try: ... yield 2 ... 1/0 ... yield 3 # never get here ... except ZeroDivisionError: ... yield 4 ... yield 5 ... raise ... except: ... yield 6 ... yield 7 # the "raise" above stops this ... except: ... yield 8 ... yield 9 ... try: ... x = 12 ... finally: ... yield 10 ... yield 11 >>> print list(f()) [1, 2, 4, 5, 8, 9, 10, 11] >>>
# A binary tree class. class Tree: def __init__(self, label, left=None, right=None): self.label = label self.left = left self.right = right def __repr__(self, level=0, indent=" "): s = level*indent + `self.label` if self.left: s = s + "\n" + self.left.__repr__(level+1, indent) if self.right: s = s + "\n" + self.right.__repr__(level+1, indent) return s def __iter__(self): return inorder(self) # Create a Tree from a list. def tree(list): n = len(list) if n == 0: return  i = n / 2 return Tree(list[i], tree(list[:i]), tree(list[i+1:])) # A recursive generator that generates Tree labels in in-order. def inorder(t): if t: for x in inorder(t.left): yield x yield t.label for x in inorder(t.right): yield x # Show it off: create a tree. t = tree("ABCDEFGHIJKLMNOPQRSTUVWXYZ") # Print the nodes of the tree in in-order. for x in t: print x, print # A non-recursive generator. def inorder(node): stack =  while node: while node.left: stack.append(node) node = node.left yield node.label while not node.right: try: node = stack.pop() except IndexError: return yield node.label node = node.right # Exercise the non-recursive generator. for x in t: print x, print
Both output blocks display:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
Q & A
Why not a new keyword instead of reusing
See BDFL Pronouncements section below.
Why a new keyword for
yield? Why not a builtin function instead?
Control flow is much better expressed via keyword in Python, and yield is a
control construct. It’s also believed that efficient implementation in Jython
requires that the compiler be able to determine potential suspension points at
compile-time, and a new keyword makes that easy. The CPython reference
implementation also exploits it heavily, to detect which functions are
generator-functions (although a new keyword in place of
def would solve
that for CPython – but people asking the “why a new keyword?” question don’t
want any new keyword).
Then why not some other special syntax without a new keyword?
For example, one of these instead of
return 3 and continue return and continue 3 return generating 3 continue return 3 return >> , 3 from generator return 3 return >> 3 return << 3 >> 3 << 3 * 3
Did I miss one <wink>? Out of hundreds of messages, I counted three
suggesting such an alternative, and extracted the above from them. It would be
nice not to need a new keyword, but nicer to make
yield very clear – I
don’t want to have to deduce that a yield is occurring from making sense of a
previously senseless sequence of keywords or operators. Still, if this
attracts enough interest, proponents should settle on a single consensus
suggestion, and Guido will Pronounce on it.
return at all? Why not force termination to be spelled
The mechanics of
StopIteration are low-level details, much like the
IndexError in Python 2.1: the implementation needs to do
something well-defined under the covers, and Python exposes these mechanisms
for advanced users. That’s not an argument for forcing everyone to work at
that level, though.
return means “I’m done” in any kind of function, and
that’s easy to explain and to use. Note that
return isn’t always equivalent
raise StopIteration in try/except construct, either (see the
“Specification: Return” section).
Then why not allow an expression on
Perhaps we will someday. In Icon,
return expr means both “I’m done”, and
“but I have one final useful value to return too, and this is it”. At the
start, and in the absence of compelling uses for
return expr, it’s simply
cleaner to use
yield exclusively for delivering values.
Introduce another new keyword (say,
generator) in place
def, or otherwise alter the syntax, to distinguish generator-functions
from non-generator functions.
In practice (how you think about them), generators are functions, but with the twist that they’re resumable. The mechanics of how they’re set up is a comparatively minor technical issue, and introducing a new keyword would unhelpfully overemphasize the mechanics of how generators get started (a vital but tiny part of a generator’s life).
In reality (how you think about them), generator-functions are actually
factory functions that produce generator-iterators as if by magic. In this
respect they’re radically different from non-generator functions, acting more
like a constructor than a function, so reusing
def is at best confusing.
yield statement buried in the body is not enough warning that the
semantics are so different.
def it stays. No argument on either side is totally convincing, so I
have consulted my language designer’s intuition. It tells me that the syntax
proposed in the PEP is exactly right - not too hot, not too cold. But, like
the Oracle at Delphi in Greek mythology, it doesn’t tell me why, so I don’t
have a rebuttal for the arguments against the PEP syntax. The best I can come
up with (apart from agreeing with the rebuttals … already made) is “FUD”.
If this had been part of the language from day one, I very much doubt it would
have made Andrew Kuchling’s “Python Warts” page.
The current implementation, in a preliminary state (no docs, but well tested and solid), is part of Python’s CVS development tree . Using this requires that you build Python from source.
This was derived from an earlier patch by Neil Schemenauer .
Footnotes and References
This document has been placed in the public domain.
Last modified: 2022-10-05 16:48:43 GMT