第825页 | Learning Python-Mark Lutz | 阅读 ‧ 电子书库

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Using 3.0 (and 2.6) bytearray Objects

So far we’ve focused on str and bytes, since they subsume Python 2’s unicode and str. Python 3.0 has a third string type, though—bytearray, a mutable sequence of integers in the range 0 through 255, is essentially a mutable variant of bytes. As such, it supports the same string methods and sequence operations as bytes, as well as many of the mutable in-place-change operations supported by lists. The bytearray type is also available in Python 2.6 as a back-port from 3.0, but it does not enforce the strict text/binary distinction there that it does in 3.0.

Let’s take a quick tour. bytearray objects may be created by calling the bytearray built-in. In Python 2.6, any string may be used to initialize:

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# Creation in 2.6: a mutable sequence of small (0..255) ints

>>> S = 'spam'
>>> C = bytearray(S) # A back-port from 3.0 in 2.6
>>> C # b'..' == '..' in 2.6 (str)
bytearray(b'spam')

In Python 3.0, an encoding name or byte string is required, because text and binary strings do not mix, though byte strings may reflect encoded Unicode text:

# Creation in 3.0: text/binary do not mix

>>> S = 'spam'
>>> C = bytearray(S)
TypeError: string argument without an encoding

>>> C = bytearray(S, 'latin1') # A content-specific type in 3.0
>>> C
bytearray(b'spam')

>>> B = b'spam' # b'..' != '..' in 3.0 (bytes/str)
>>> C = bytearray(B)
>>> C
bytearray(b'spam')

Once created, bytearray objects are sequences of small integers like bytes and are mutable like lists, though they require an integer for index assignments, not a string (all of the following is a continuation of this session and is run under Python 3.0 unless otherwise noted—see comments for 2.6 usage notes):

# Mutable, but must assign ints, not strings

>>> C[0]
115

>>> C[0] = 'x' # This and the next work in 2.6
TypeError: an integer is required

>>> C[0] = b'x'
TypeError: an integer is required

>>> C[0] = ord('x')
>>> C
bytearray(b'xpam')

>>> C[1] = b'Y'[0]
>>> C
bytearray(b'xYam')

Processing bytearray objects borrows from both strings and lists, since they are mutable byte strings. Besides named methods, the __iadd__ and __setitem__ methods in bytearray implement += in-place concatenation and index assignment, respectively:

# Methods overlap with both str and bytes, but also has list's mutable methods

>>> set(dir(b'abc')) - set(dir(bytearray(b'abc')))
{'__getnewargs__'}

>>> set(dir(bytearray(b'abc'))) - set(dir(b'abc'))
{'insert', '__alloc__', 'reverse', 'extend', '__delitem__', 'pop', '__setitem__'
, '__iadd__', 'remove', 'append', '__imul__'}

You can change a bytearray in-place with both index assignment, as you’ve just seen, and list-like methods like those shown here (to change text in-place in 2.6, you would need to convert to and then from a list, with list(str) and ''.join(list)):

# Mutable method calls

>>> C
bytearray(b'xYam')

>>> C.append(b'LMN') # 2.6 requires string of size 1
TypeError: an integer is required

>>> C.append(ord('L'))
>>> C
bytearray(b'xYamL')

>>> C.extend(b'MNO')
>>> C
bytearray(b'xYamLMNO')

All the usual sequence operations and string methods work on bytearrays, as you would expect (notice that like bytes objects, their expressions and methods expect bytes arguments, not str arguments):

# Sequence operations and string methods

>>> C + b'!#'
bytearray(b'xYamLMNO!#')

>>> C[0]
120

>>> C[1:]
bytearray(b'YamLMNO')

>>> len(C)
8

>>> C
bytearray(b'xYamLMNO')

>>> C.replace('xY', 'sp') # This works in 2.6
TypeError: Type str doesn't support the buffer API

>>> C.replace(b'xY', b'sp')
bytearray(b'spamLMNO')

>>> C
bytearray(b'xYamLMNO')

>>> C * 4
bytearray(b'xYamLMNOxYamLMNOxYamLMNOxYamLMNO')

Finally, by way of summary, the following examples demonstrate how bytes and bytearray objects are sequences of ints, and str objects are sequences of characters:

# Binary versus text

>>> B # B is same as S in 2.6
b'spam'
>>> list(B)
[115, 112, 97, 109]

>>> C
bytearray(b'xYamLMNO')
>>> list(C)
[120, 89, 97, 109, 76, 77, 78, 79]

>>> S
'spam'
>>> list(S)
['s', 'p', 'a', 'm']