224 lines
6.5 KiB
Plaintext
224 lines
6.5 KiB
Plaintext
PEP: 3140
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Title: str(container) should call str(item), not repr(item)
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Version: $Revision$
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Last-Modified: $Date$
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Author: Oleg Broytman <phd@phdru.name>,
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Jim J. Jewett <jimjjewett@gmail.com>
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Discussions-To: python-3000@python.org
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Status: Rejected
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 27-May-2008
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Post-History: 28-May-2008
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Rejection
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=========
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Guido said this would cause too much disturbance too close to beta. See [1]_.
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Abstract
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========
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This document discusses the advantages and disadvantages of the
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current implementation of ``str(container)``. It also discusses the
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pros and cons of a different approach - to call ``str(item)`` instead
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of ``repr(item)``.
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Motivation
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==========
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Currently ``str(container)`` calls ``repr`` on items. Arguments for it:
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* containers refuse to guess what the user wants to see on
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``str(container)`` - surroundings, delimiters, and so on;
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* ``repr(item)`` usually displays type information - apostrophes
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around strings, class names, etc.
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Arguments against:
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* it's illogical; ``str()`` is expected to call ``__str__`` if it exists,
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not ``__repr__``;
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* there is no standard way to print a container's content calling
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items' ``__str__``, that's inconvenient in cases where ``__str__`` and
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``__repr__`` return different results;
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* ``repr(item)`` sometimes do wrong things (hex-escapes non-ASCII strings,
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e.g.)
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This PEP proposes to change how ``str(container)`` works. It is
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proposed to mimic how ``repr(container)`` works except one detail - call
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``str`` on items instead of ``repr``. This allows a user to choose
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what results she want to get - from ``item.__repr__`` or ``item.__str__``.
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Current situation
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=================
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Most container types (tuples, lists, dicts, sets, etc.) do not
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implement ``__str__`` method, so ``str(container)`` calls
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``container.__repr__``, and ``container.__repr__``, once called, forgets
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it is called from ``str`` and always calls ``repr`` on the container's
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items.
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This behaviour has advantages and disadvantages. One advantage is
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that most items are represented with type information - strings
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are surrounded by apostrophes, instances may have both class name
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and instance data::
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>>> print([42, '42'])
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[42, '42']
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>>> print([Decimal('42'), datetime.now()])
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[Decimal("42"), datetime.datetime(2008, 5, 27, 19, 57, 43, 485028)]
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The disadvantage is that ``__repr__`` often returns technical data
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(like '``<object at address>``') or unreadable string (hex-encoded
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string if the input is non-ASCII string)::
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>>> print(['тест'])
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['\xd4\xc5\xd3\xd4']
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One of the motivations for :pep:`3138` is that neither ``repr`` nor ``str``
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will allow the sensible printing of dicts whose keys are non-ASCII
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text strings. Now that Unicode identifiers are allowed, it
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includes Python's own attribute dicts. This also includes JSON
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serialization (and caused some hoops for the json lib).
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:pep:`3138` proposes to fix this by breaking the "repr is safe ASCII"
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invariant, and changing the way ``repr`` (which is used for
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persistence) outputs some objects, with system-dependent failures.
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Changing how ``str(container)`` works would allow easy debugging in
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the normal case, and retain the safety of ASCII-only for the
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machine-readable case. The only downside is that ``str(x)`` and
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``repr(x)`` would more often be different -- but only in those cases
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where the current almost-the-same version is insufficient.
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It also seems illogical that ``str(container)`` calls ``repr`` on items
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instead of ``str``. It's only logical to expect following code::
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class Test:
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def __str__(self):
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return "STR"
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def __repr__(self):
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return "REPR"
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test = Test()
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print(test)
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print(repr(test))
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print([test])
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print(str([test]))
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to print::
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STR
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REPR
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[STR]
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[STR]
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where it actually prints::
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STR
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REPR
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[REPR]
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[REPR]
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Especially it is illogical to see that print in Python 2 uses ``str``
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if it is called on what seems to be a tuple::
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>>> print Decimal('42'), datetime.now()
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42 2008-05-27 20:16:22.534285
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where on an actual tuple it prints::
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>>> print((Decimal('42'), datetime.now()))
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(Decimal("42"), datetime.datetime(2008, 5, 27, 20, 16, 27, 937911))
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A different approach - call ``str(item)``
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=========================================
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For example, with numbers it is often only the value that people
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care about.
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::
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>>> print Decimal('3')
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3
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But putting the value in a list forces users to read the type
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information, exactly as if ``repr`` had been called for the benefit of
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a machine::
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>>> print [Decimal('3')]
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[Decimal("3")]
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After this change, the type information would not clutter the ``str``
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output::
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>>> print "%s".format([Decimal('3')])
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[3]
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>>> str([Decimal('3')]) # ==
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[3]
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But it would still be available if desired::
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>>> print "%r".format([Decimal('3')])
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[Decimal('3')]
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>>> repr([Decimal('3')]) # ==
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[Decimal('3')]
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There is a number of strategies to fix the problem. The most
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radical is to change ``__repr__`` so it accepts a new parameter (flag)
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"called from ``str``, so call ``str`` on items, not ``repr``". The
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drawback of the proposal is that every ``__repr__`` implementation
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must be changed. Introspection could help a bit (inspect ``__repr__``
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before calling if it accepts 2 or 3 parameters), but introspection
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doesn't work on classes written in C, like all built-in containers.
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Less radical proposal is to implement ``__str__`` methods for built-in
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container types. The obvious drawback is a duplication of effort - all
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those ``__str__`` and ``__repr__`` implementations are only differ
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in one small detail - if they call ``str`` or ``repr`` on items.
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The most conservative proposal is not to change str at all but
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to allow developers to implement their own application- or
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library-specific pretty-printers. The drawback is again
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a multiplication of effort and proliferation of many small
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specific container-traversal algorithms.
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Backward compatibility
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======================
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In those cases where type information is more important than
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usual, it will still be possible to get the current results by
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calling ``repr`` explicitly.
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References
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==========
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.. [1] Guido van Rossum, PEP: str(container) should call str(item), not
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repr(item)
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https://mail.python.org/pipermail/python-3000/2008-May/013876.html
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Copyright
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=========
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This document has been placed in the public domain.
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..
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Local Variables:
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mode: indented-text
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indent-tabs-mode: nil
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sentence-end-double-space: t
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fill-column: 70
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coding: utf-8
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End:
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