.. highlightlang:: c
+.. index::
+ single: buffer protocol
+ single: buffer interface; (see buffer protocol)
+ single: buffer object; (see buffer protocol)
+
.. _bufferobjects:
Buffer Protocol
.. sectionauthor:: Stefan Krah
-.. index::
- single: buffer interface
-
Certain objects available in Python wrap access to an underlying memory
array or *buffer*. Such objects include the built-in :class:`bytes` and
:class:`bytearray`, and some extension types like :class:`array.array`.
then desirable, in some situations, to access that buffer directly and
without intermediate copying.
-Python provides such a facility at the C level in the form of the *buffer
-protocol*. This protocol has two sides:
+Python provides such a facility at the C level in the form of the :ref:`buffer
+protocol <bufferobjects>`. This protocol has two sides:
.. index:: single: PyBufferProcs
is used by most built-in types: :ref:`formatspec`.
The default *format_spec* is an empty string which usually gives the same
- effect as calling ``str(value)``.
+ effect as calling :func:`str(value) <str>`.
A call to ``format(value, format_spec)`` is translated to
``type(value).__format__(format_spec)`` which bypasses the instance
For more information on static methods, consult the documentation on the
standard type hierarchy in :ref:`types`.
+ .. index::
+ single: string; str() (built-in function)
+
.. _func-str:
.. function:: str(object='')
- str(object[, encoding[, errors]])
-
- Return a :ref:`string <textseq>` version of an object, using one of the
- following modes:
-
- If *encoding* and/or *errors* are given, :func:`str` will decode the
- *object* which can either be a byte string or a character buffer using
- the codec for *encoding*. The *encoding* parameter is a string giving
- the name of an encoding; if the encoding is not known, :exc:`LookupError`
- is raised. Error handling is done according to *errors*; this specifies the
- treatment of characters which are invalid in the input encoding. If
- *errors* is ``'strict'`` (the default), a :exc:`ValueError` is raised on
- errors, while a value of ``'ignore'`` causes errors to be silently ignored,
- and a value of ``'replace'`` causes the official Unicode replacement character,
- U+FFFD, to be used to replace input characters which cannot be decoded.
- See also the :mod:`codecs` module.
-
- When only *object* is given, this returns its nicely printable representation.
- For strings, this is the string itself. The difference with ``repr(object)``
- is that ``str(object)`` does not always attempt to return a string that is
- acceptable to :func:`eval`; its goal is to return a printable string.
- With no arguments, this returns the empty string.
-
- Objects can specify what ``str(object)`` returns by defining a :meth:`__str__`
- special method.
-
- For more information on strings and string methods, see the :ref:`textseq`
- section. To output formatted strings, see the :ref:`string-formatting`
+ str(object=b'', encoding='utf-8', errors='strict')
+
+ Return a :ref:`string <textseq>` version of *object*. If *object* is not
+ provided, returns the empty string. Otherwise, the behavior of ``str()``
+ depends on whether *encoding* or *errors* is given, as follows.
+
+ If neither *encoding* nor *errors* is given, ``str(object)`` returns
+ :meth:`object.__str__() <object.__str__>`, which is the "informal" or nicely
+ printable string representation of *object*. For string objects, this is
+ the string itself. If *object* does not have a :meth:`~object.__str__`
+ method, then :func:`str` falls back to returning
+ :meth:`repr(object) <repr>`.
+
+ .. index::
+ single: buffer protocol; str() (built-in function)
+ single: bytes; str() (built-in function)
+
+ If at least one of *encoding* or *errors* is given, *object* should be a
+ :class:`bytes` or :class:`bytearray` object, or more generally any object
+ that supports the :ref:`buffer protocol <bufferobjects>`. In this case, if
+ *object* is a :class:`bytes` (or :class:`bytearray`) object, then
+ ``str(bytes, encoding, errors)`` is equivalent to
+ :meth:`bytes.decode(encoding, errors) <bytes.decode>`. Otherwise, the bytes
+ object underlying the buffer object is obtained before calling
+ :meth:`bytes.decode`. See :ref:`binaryseq` and
+ :ref:`bufferobjects` for information on buffer objects.
+
+ Passing a :class:`bytes` object to :func:`str` without the *encoding*
+ or *errors* arguments falls under the first case of returning the informal
+ string representation (see also the :option:`-b` command-line option to
+ Python). For example::
+
+ >>> str(b'Zoot!')
+ "b'Zoot!'"
+
+ ``str`` is a built-in :term:`type`. For more information on the string
+ type and its methods, see the :ref:`textseq` and :ref:`string-methods`
+ sections. To output formatted strings, see the :ref:`string-formatting`
section. In addition, see the :ref:`stringservices` section.
The :attr:`start`, :attr:`stop` and :attr:`step` attributes.
+.. index::
+ single: string; text sequence type
+ single: str() (built-in function); (see also string)
+ object: string
+
.. _textseq:
Text Sequence Type --- :class:`str`
===================================
-.. index::
- object: string
- object: bytes
- object: bytearray
- object: io.StringIO
-
-
-Textual data in Python is handled with ``str`` objects, which are immutable
+Textual data in Python is handled with :class:`str` objects, or :dfn:`strings`.
+Strings are immutable
:ref:`sequences <typesseq>` of Unicode code points. String literals are
written in a variety of ways:
Since there is no separate "character" type, indexing a string produces
strings of length 1. That is, for a non-empty string *s*, ``s[0] == s[0:1]``.
+.. index::
+ object: io.StringIO
+
There is also no mutable string type, but :meth:`str.join` or
:class:`io.StringIO` can be used to efficiently construct strings from
multiple fragments.
longer replaced by ``%g`` conversions.
+.. index::
+ single: buffer protocol; binary sequence types
+
.. _binaryseq:
Binary Sequence Types --- :class:`bytes`, :class:`bytearray`, :class:`memoryview`
The core built-in types for manipulating binary data are :class:`bytes` and
:class:`bytearray`. They are supported by :class:`memoryview` which uses
-the buffer protocol to access the memory of other binary objects without
-needing to make a copy.
+the :ref:`buffer protocol <bufferobjects>` to access the memory of other
+binary objects without needing to make a copy.
The :mod:`array` module supports efficient storage of basic data types like
32-bit integers and IEEE754 double-precision floating values.
modules are still available at the time when the :meth:`__del__` method is
called.
+ .. index::
+ single: repr() (built-in function); __repr__() (object method)
-.. method:: object.__repr__(self)
- .. index:: builtin: repr
+.. method:: object.__repr__(self)
Called by the :func:`repr` built-in function to compute the "official" string
representation of an object. If at all possible, this should look like a
This is typically used for debugging, so it is important that the representation
is information-rich and unambiguous.
+ .. index::
+ single: string; __str__() (object method)
+ single: format() (built-in function); __str__() (object method)
+ single: print() (built-in function); __str__() (object method)
+
.. method:: object.__str__(self)
- .. index::
- builtin: str
- builtin: print
+ Called by :func:`str(object) <str>` and the built-in functions
+ :func:`format` and :func:`print` to compute the "informal" or nicely
+ printable string representation of an object. The return value must be a
+ :ref:`string <textseq>` object.
- Called by the :func:`str` built-in function and by the :func:`print` function
- to compute the "informal" string representation of an object. This differs
- from :meth:`__repr__` in that it does not have to be a valid Python
- expression: a more convenient or concise representation may be used instead.
- The return value must be a string object.
+ This method differs from :meth:`object.__repr__` in that there is no
+ expectation that :meth:`__str__` return a valid Python expression: a more
+ convenient or concise representation can be used.
+
+ The default implementation defined by the built-in type :class:`object`
+ calls :meth:`object.__repr__`.
.. XXX what about subclasses of string?
self.assertRaises(TypeError, setattr, sys, 1, 'spam')
self.assertRaises(TypeError, setattr)
+ # test_str(): see test_unicode.py and test_bytes.py for str() tests.
def test_sum(self):
self.assertEqual(sum([]), 0)
self.assertRaises(TypeError, str, 42, 42, 42)
+ def test_constructor_keyword_args(self):
+ """Pass various keyword argument combinations to the constructor."""
+ # The object argument can be passed as a keyword.
+ self.assertEqual(str(object='foo'), 'foo')
+ self.assertEqual(str(object=b'foo', encoding='utf-8'), 'foo')
+ # The errors argument without encoding triggers "decode" mode.
+ self.assertEqual(str(b'foo', errors='strict'), 'foo') # not "b'foo'"
+ self.assertEqual(str(object=b'foo', errors='strict'), 'foo')
+
+ def test_constructor_defaults(self):
+ """Check the constructor argument defaults."""
+ # The object argument defaults to '' or b''.
+ self.assertEqual(str(), '')
+ self.assertEqual(str(errors='strict'), '')
+ utf8_cent = '¢'.encode('utf-8')
+ # The encoding argument defaults to utf-8.
+ self.assertEqual(str(utf8_cent, errors='strict'), '¢')
+ # The errors argument defaults to strict.
+ self.assertRaises(UnicodeDecodeError, str, utf8_cent, encoding='ascii')
+
def test_codecs_utf7(self):
utfTests = [
('A\u2262\u0391.', b'A+ImIDkQ.'), # RFC2152 example
Documentation
-------------
+- Issue #13538: Improve str() and object.__str__() documentation.
+
- Issue #16489: Make it clearer that importlib.find_loader() requires any and
all packages to be separately imported.
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