What does the 'b' character do in front of a string literal?
Python 3.x makes a clear distinction between the types:
str
='...'
literals = a sequence of Unicode characters (Latin-1, UCS-2 or UCS-4, depending on the widest character in the string)bytes
=b'...'
literals = a sequence of octets (integers between 0 and 255)
If you're familiar with:
- Java or C#, think of
str
asString
andbytes
asbyte[]
; - SQL, think of
str
asNVARCHAR
andbytes
asBINARY
orBLOB
; - Windows registry, think of
str
asREG_SZ
andbytes
asREG_BINARY
.
If you're familiar with C(++), then forget everything you've learned about char
and strings, because a character is not a byte. That idea is long obsolete.
You use str
when you want to represent text.
print('שלום עולם')
You use bytes
when you want to represent low-level binary data like structs.
NaN = struct.unpack('>d', b'\xff\xf8\x00\x00\x00\x00\x00\x00')[0]
You can encode a str
to a bytes
object.
>>> '\uFEFF'.encode('UTF-8')
b'\xef\xbb\xbf'
And you can decode a bytes
into a str
.
>>> b'\xE2\x82\xAC'.decode('UTF-8')
'€'
But you can't freely mix the two types.
>>> b'\xEF\xBB\xBF' + 'Text with a UTF-8 BOM'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can't concat bytes to str
The b'...'
notation is somewhat confusing in that it allows the bytes 0x01-0x7F to be specified with ASCII characters instead of hex numbers.
>>> b'A' == b'\x41'
True
But I must emphasize, a character is not a byte.
>>> 'A' == b'A'
False
In Python 2.x
Pre-3.0 versions of Python lacked this kind of distinction between text and binary data. Instead, there was:
unicode
=u'...'
literals = sequence of Unicode characters = 3.xstr
str
='...'
literals = sequences of confounded bytes/characters- Usually text, encoded in some unspecified encoding.
- But also used to represent binary data like
struct.pack
output.
In order to ease the 2.x-to-3.x transition, the b'...'
literal syntax was backported to Python 2.6, in order to allow distinguishing binary strings (which should be bytes
in 3.x) from text strings (which should be str
in 3.x). The b
prefix does nothing in 2.x, but tells the 2to3
script not to convert it to a Unicode string in 3.x.
So yes, b'...'
literals in Python have the same purpose that they do in PHP.
Also, just out of curiosity, are there more symbols than the b and u that do other things?
The r
prefix creates a raw string (e.g., r'\t'
is a backslash + t
instead of a tab), and triple quotes '''...'''
or """..."""
allow multi-line string literals.
To quote the Python 2.x documentation:
A prefix of 'b' or 'B' is ignored in Python 2; it indicates that the literal should become a bytes literal in Python 3 (e.g. when code is automatically converted with 2to3). A 'u' or 'b' prefix may be followed by an 'r' prefix.
The Python 3 documentation states:
Bytes literals are always prefixed with 'b' or 'B'; they produce an instance of the bytes type instead of the str type. They may only contain ASCII characters; bytes with a numeric value of 128 or greater must be expressed with escapes.