What does the 'b' character do in front of a string literal?
Python 3.x makes a clear distinction between the types:
'...'literals = a sequence of Unicode characters (Latin-1, UCS-2 or UCS-4, depending on the widest character in the string)
b'...'literals = a sequence of octets (integers between 0 and 255)
If you're familiar with:
- Java or C#, think of
- SQL, think of
- Windows registry, think of
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.
str when you want to represent text.
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')
You can encode a
str to a
And you can decode a
bytes into a
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
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:
u'...'literals = sequence of Unicode characters = 3.x
'...'literals = sequences of confounded bytes/characters
- Usually text, encoded in some unspecified encoding.
- But also used to represent binary data like
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.
b'...' literals in Python have the same purpose that they do in PHP.
Also, just out of curiosity, are theremore symbols than the b and u that doother things?
r prefix creates a raw string (e.g.,
r'\t' is a backslash +
t instead of a tab), and triple quotes
"""...""" 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.
The b denotes a byte string.
Bytes are the actual data. Strings are an abstraction.
If you had multi-character string object and you took a single character, it would be a string, and it might be more than 1 byte in size depending on encoding.
If took 1 byte with a byte string, you'd get a single 8-bit value from 0-255 and it might not represent a complete character if those characters due to encoding were > 1 byte.
TBH I'd use strings unless I had some specific low level reason to use bytes.