python-peps/pep-0234.txt

PEP: 234
Title: Iterators
Version: $Revision$
Author: ping@lfw.org (Ka-Ping Yee), guido@python.org (Guido van Rossum)
Status: Draft
Type: Standards Track
Python-Version: 2.1
Created: 30-Jan-2001
Post-History: 30-Apr-2001

Abstract

    This document proposes an iteration interface that objects can
    provide to control the behaviour of 'for' loops.  Looping is
    customized by providing a method that produces an iterator object.
    The iterator provides a 'get next value' operation that produces
    the next item in the sequence each time it is called, raising an
    exception when no more items are available.

    In addition, specific iterators over the keys of a dictionary and
    over the lines of a file are proposed, and a proposal is made to
    allow spelling dict.has_key(key) as "key in dict".

    Note: this is an almost complete rewrite of this PEP by the second
    author, describing the actual implementation checked into the
    trunk of the Python 2.2 CVS tree.  It is still open for
    discussion.  Some of the more esoteric proposals in the original
    version of this PEP have been withdrawn for now; these may be the
    subject of a separate PEP in the future.


C API Specification

    A new exception is defined, StopIteration, which can be used to
    signal the end of an iteration.

    A new slot named tp_iter for requesting an iterator is added to
    the type object structure.  This should be a function of one
    PyObject * argument returning a PyObject *, or NULL.  To use this
    slot, a new C API function PyObject_GetIter() is added, with the
    same signature as the tp_iter slot function.

    Another new slot, named tp_iternext, is added to the type
    structure, for obtaining the next value in the iteration.  To use
    this slot, a new C API function PyIter_Next() is added.  The
    signature for both the slot and the API function is as follows:
    the argument is a PyObject * and so is the return value.  When the
    return value is non-NULL, it is the next value in the iteration.
    When it is NULL, there are three possibilities:

    - No exception is set; this implies the end of the iteration.

    - The StopIteration exception (or a derived exception class) is
      set; this implies the end of the iteration.

    - Some other exception is set; this means that an error occurred
      that should be propagated normally.

    In addition to the tp_iternext slot, every iterator object must
    also implement a next() method, callable without arguments.  This
    should have the same semantics as the tp_iternext slot function,
    except that the only way to signal the end of the iteration is to
    raise StopIteration.  The iterator object should not care whether
    its tp_iternext slot function is called or its next() method, and
    the caller may mix calls arbitrarily.  (The next() method is for
    the benefit of Python code using iterators directly; the
    tp_iternext slot is added to make 'for' loops more efficient.)

    To ensure binary backwards compatibility, a new flag
    Py_TPFLAGS_HAVE_ITER is added to the set of flags in the tp_flags
    field, and to the default flags macro.  This flag must be tested
    before accessing the tp_iter or tp_iternext slots.  The macro
    PyIter_Check() tests whether an object has the appropriate flag
    set and has a non-NULL tp_iternext slot.  There is no such macro
    for the tp_iter slot (since the only place where this slot is
    referenced should be PyObject_GetIter()).

    (Note: the tp_iter slot can be present on any object; the
    tp_iternext slot should only be present on objects that act as
    iterators.)

    For backwards compatibility, the PyObject_GetIter() function
    implements fallback semantics when its argument is a sequence that
    does not implement a tp_iter function: a lightweight sequence
    iterator object is constructed in that case which iterates over
    the items of the sequence in the natural order.

    The Python bytecode generated for 'for' loops is changed to use
    new opcodes, GET_ITER and FOR_ITER, that use the iterator protocol
    rather than the sequence protocol to get the next value for the
    loop variable.  This makes it possible to use a 'for' loop to loop
    over non-sequence objects that support the tp_iter slot.  Other
    places where the interpreter loops over the values of a sequence
    should also be changed to use iterators.

    Iterators ought to implement the tp_iter slot as returning a
    reference to themselves; this is needed to make it possible to
    use an iterator (as opposed to a sequence) in a for loop.


Python API Specification

    The StopIteration exception is made visible as one of the
    standard exceptions.  It is derived from Exception.

    A new built-in function is defined, iter(), which can be called in
    two ways:

    - iter(obj) calls PyObject_GetIter(obj).

    - iter(callable, sentinel) returns a special kind of iterator that
      calls the callable to produce a new value, and compares the
      return value to the sentinel value.  If the return value equals
      the sentinel, this signals the end of the iteration and
      StopIteration is raised rather than returning normal; if the
      return value does not equal the sentinel, it is returned as the
      next value from the iterator.  If the callable raises an
      exception, this is propagated normally; in particular, the
      function is allowed to raise StopIteration as an alternative way
      to end the iteration.  (This functionality is available from the
      C API as PyCallIter_New(callable, sentinel).)

    Iterator objects returned by either form of iter() have a next()
    method.  This method either returns the next value in the
    iteration, or raises StopIteration (or a derived exception class)
    to signal the end of the iteration.  Any other exception should be
    considered to signify an error and should be propagated normally,
    not taken to mean the end of the iteration.

    Classes can define how they are iterated over by defining an
    __iter__() method; this should take no additional arguments and
    return a valid iterator object.  A class is a valid iterator
    object when it defines a next() method that behaves as described
    above.  A class that wants to be an iterator also ought to
    implement __iter__() returning itself.


Dictionary Iterators

    The following two proposals are somewhat controversial.  They are
    also independent from the main iterator implementation.  However,
    they are both very useful.

    - Dictionaries implement a sq_contains slot that implements the
      same test as the has_key() method.  This means that we can write

          if k in dict: ...

      which is equivalent to

          if dict.has_key(k): ...

    - Dictionaries implement a tp_iter slot that returns an efficient
      iterator that iterates over the keys of the dictionary.  During
      such an iteration, the dictionary should not be modified, except
      that setting the value for an existing key is allowed (deletions
      or additions are not, nor is the update() method).  This means
      that we can write

          for k in dict: ...

      which is equivalent to, but much faster than

          for k in dict.keys(): ...

      as long as the restriction on modifications to the dictionary
      (either by the loop or by another thread) are not violated.

    - Add methods to dictionaries that return different kinds of
      iterators explicitly:

          for key in dict.iterkeys(): ...

          for value in dict.itervalues(): ...

          for key, value in dict.iteritems(): ...

      This means that "for x in dict" is shorthand for "for x in
      dict.iterkeys()".

    If this proposal is accepted, it makes sense to recommend that
    other mappings, if they support iterators at all, should also
    iterate over the keys.  However, this should not be taken as an
    absolute rule; specific applications may have different
    requirements.


File Iterators

    The following proposal is not controversial, but should be
    considered a separate step after introducing the iterator
    framework described above.  It is useful because it provides us
    with a good answer to the complaint that the common idiom to
    iterate over the lines of a file is ugly and slow.

    - Files implement a tp_iter slot that is equivalent to
      iter(f.readline, "").  This means that we can write

          for line in file:
              ...

      as a shorthand for

          for line in iter(file.readline, ""):
              ...

      which is equivalent to, but faster than

          while 1:
              line = file.readline()
              if not line:
                  break
              ...

    This also shows that some iterators are destructive: they consume
    all the values and a second iterator cannot easily be created that
    iterates independently over the same values.  You could open the
    file for a second time, or seek() to the beginning, but these
    solutions don't work for all file types, e.g. they don't work when
    the open file object really represents a pipe or a stream socket.


Rationale

    If all the parts of the proposal are included, this addresses many
    concerns in a consistent and flexible fashion.  Among its chief
    virtues are the following four -- no, five -- no, six -- points:

    1. It provides an extensible iterator interface.

    2. It allows performance enhancements to list iteration.

    3. It allows big performance enhancements to dictionary iteration.

    4. It allows one to provide an interface for just iteration
       without pretending to provide random access to elements.

    5. It is backward-compatible with all existing user-defined
       classes and extension objects that emulate sequences and
       mappings, even mappings that only implement a subset of
       {__getitem__, keys, values, items}.

    6. It makes code iterating over non-sequence collections more
       concise and readable.


Open Issues

    The following questions are still open.

    - The name iter() is an abbreviation.  Alternatives proposed
      include iterate(), traverse(), but these appear too long.
      Python has a history of using abbrs for common builtins,
      e.g. repr(), str(), len().

    - Using the same name for two different operations (getting an
      iterator from an object and making an iterator for a function
      with an sentinel value) is somewhat ugly.  I haven't seen a
      better name for the second operation though, and since they both
      return an iterator, it's easy to remember.

    - Once a particular iterator object has raised StopIteration, will
      it also raise StopIteration on all subsequent next() calls?
      Some say that it would be useful to require this, others say
      that it is useful to leave this open to individual iterators.
      Note that this may require an additional state bit for some
      iterator implementations (e.g. function-wrapping iterators).

    - It has been proposed that a file object should be its own
      iterator, with a next() method returning the next line.  This
      has certain advantages, and makes it even clearer that this
      iterator is destructive.  The disadvantage is that this would
      make it even more painful to implement the "sticky
      StopIteration" feature proposed in the previous bullet.

    - Some folks have requested extensions of the iterator protocol,
      e.g. prev() to get the previous item, current() to get the
      current item again, finished() to test whether the iterator is
      finished, and maybe even others, like rewind(), __len__(),
      position().

      While some of these are useful, many of these cannot easily be
      implemented for all iterator types without adding arbitrary
      buffering, and sometimes they can't be implemented at all (or
      not reasonably).  E.g. anything to do with reversing directions
      can't be done when iterating over a file or function.  Maybe a
      separate PEP can be drafted to standardize the names for such
      operations when the are implementable.

    - There is still discussion about whether

          for x in dict: ...

      should assign x the successive keys, values, or items of the
      dictionary.  The symmetry between "if x in y" and "for x in y"
      suggests that it should iterate over keys.  This symmetry has been
      observed by many independently and has even been used to "explain"
      one using the other.  This is because for sequences, "if x in y"
      iterates over y comparing the iterated values to x.  If we adopt
      both of the above proposals, this will also hold for
      dictionaries.

      The argument against making "for x in dict" iterate over the keys
      comes mostly from a practicality point of view: scans of the
      standard library show that there are about as many uses of "for x
      in dict.items()" as there are of "for x in dict.keys()", with the
      items() version having a small majority.  Presumably many of the
      loops using keys() use the corresponding value anyway, by writing
      dict[x], so (the argument goes) by making both the key and value
      available, we could support the largest number of cases.  While
      this is true, I (Guido) find the correspondence between "for x in
      dict" and "if x in dict" too compelling to break, and there's not
      much overhead in having to write dict[x] to explicitly get the
      value.

      For fast iteration over items, use "for key, value in
      dict.iteritems()".  I've timed the difference between

          for key in dict: dict[key]

      and

          for key, value in dict.iteritems(): pass

      and found that the latter is only about 7% faster.


Resolved Issues

    The following topics have been decided by consensus or BDFL
    pronouncement.

    - Two alternative spellings for next() have been proposed but
      rejected: __next__(), because it corresponds to a type object
      slot (tp_iternext); and __call__(), because this is the only
      operation.

      Arguments against __next__(): while many iterators are used in
      for loops, it is expected that user code will also call next()
      directly, so having to write __next__() is ugly; also, a
      possible extension of the protocol would be to allow for prev(),
      current() and reset() operations; surely we don't want to use
      __prev__(), __current__(), __reset__().

      Arguments against __call__() (the original proposal): taken out
      of context, x() is not very readable, while x.next() is clear;
      there's a danger that every special-purpose object wants to use
      __call__() for its most common operation, causing more confusion
      than clarity.

    - Some folks have requested the ability to restart an iterator.
      This should be dealt with by calling iter() on a sequence
      repeatedly, not by the iterator protocol itself.

    - It has been questioned whether an exception to signal the end of
      the iteration isn't too expensive.  Several alternatives for the
      StopIteration exception have been proposed: a special value End
      to signal the end, a function end() to test whether the iterator
      is finished, even reusing the IndexError exception.

      - A special value has the problem that if a sequence ever
        contains that special value, a loop over that sequence will
        end prematurely without any warning.  If the experience with
        null-terminated C strings hasn't taught us the problems this
        can cause, imagine the trouble a Python introspection tool
        would have iterating over a list of all built-in names,
        assuming that the special End value was a built-in name!

      - Calling an end() function would require two calls per
        iteration.  Two calls is much more expensive than one call
        plus a test for an exception.  Especially the time-critical
        for loop can test very cheaply for an exception.

      - Reusing IndexError can cause confusion because it can be a
        genuine error, which would be masked by ending the loop
        prematurely.

    - Some have asked for a standard iterator type.  Presumably all
      iterators would have to be derived from this type.  But this is
      not the Python way: dictionaries are mappings because they
      support __getitem__() and a handful other operations, not
      because they are derived from an abstract mapping type.

    - Regarding "if key in dict": there is no doubt that the
      dict.has_keys(x) interpretation of "x in dict" is by far the
      most useful interpretation, probably the only useful one.  There
      has been resistance against this because "x in list" checks
      whether x is present among the values, while the proposal makes
      "x in dict" check whether x is present among the keys.  Given
      that the symmetry between lists and dictionaries is very weak,
      this argument does not have much weight.


Mailing Lists

    The iterator protocol has been discussed extensively in a mailing
    list on SourceForge:

        http://lists.sourceforge.net/lists/listinfo/python-iterators

    Initially, some of the discussion was carried out at Yahoo;
    archives are still accessible:

        http://groups.yahoo.com/group/python-iter


Copyright

    This document is in the public domain.



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