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How to convert string to bytes in Python 3

TypeError: 'str' does not support the buffer interface suggests two possible methods to convert a string to bytes:

b = bytes(mystring, 'utf-8')
b = mystring.encode('utf-8')

What are the differences between them? Which one should I opt for and why?

_{See Convert bytes to a string in Python 3 for the other way around.}

Answer*

If you look at the docs for `bytes`, it points you to [`bytearray`](https://docs.python.org/3/library/functions.html#func-bytearray):

> bytearray([source[, encoding[, errors]]])
>
>Return a new array of bytes. The bytearray type is a mutable sequence of integers in the range 0 &lt;= x &lt; 256. It has most of the usual methods of mutable sequences, described in Mutable Sequence Types, as well as most methods that the bytes type has, see Bytes and Byte Array Methods.
>
>The optional source parameter can be used to initialize the array in a few different ways:
>
>__If it is a string, you must also give the encoding (and optionally, errors) parameters; bytearray() then converts the string to bytes using str.encode().__
>
>__If it is an integer, the array will have that size and will be initialized with null bytes.__
>
>__If it is an object conforming to the buffer interface, a read-only buffer of the object will be used to initialize the bytes array.__
>
>__If it is an iterable, it must be an iterable of integers in the range 0 <= x < 256, which are used as the initial contents of the array.__
>
> __Without an argument, an array of size 0 is created.__

So `bytes` can do much more than just encode a string. It's Pythonic that it would allow you to call the constructor with any type of source parameter that makes sense. 

For encoding a string, I think that `some_string.encode(encoding)` is more Pythonic than using the constructor, because it is the most self documenting -- "take this string and encode it with this encoding" is clearer than `bytes(some_string, encoding)` -- there is no explicit verb when you use the constructor.

I checked the Python source. If you pass a unicode string to `bytes` using CPython, it calls [PyUnicode_AsEncodedString](http://hg.python.org/cpython/file/5a12416890c0/Objects/unicodeobject.c#l2328), which is the implementation of `encode`; so you're just skipping a level of indirection if you call `encode` yourself.

Also, see Serdalis' comment -- `unicode_string.encode(encoding)` is also more Pythonic because its inverse is `byte_string.decode(encoding)` and symmetry is nice.

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139

+1 for having a good argument and quotes from the python docs. Also unicode_string.encode(encoding) matches nicely with bytearray.decode(encoding) when you want your string back.

Serdalis
– Serdalis

2011年09月28日 15:30:32 +00:00
Commented Sep 28, 2011 at 15:30
12

bytearray is used when you need a mutable object. You don't need it for simple str↔bytes conversions.

hamstergene
– hamstergene

2011年09月28日 15:41:42 +00:00
Commented Sep 28, 2011 at 15:41
9

@EugeneHomyakov This has nothing to do with bytearray except that the docs for bytes don't give details, they just say "this is an immutable version of bytearray" so I have to quote from there.

agf
– agf

2011年09月28日 15:43:53 +00:00
Commented Sep 28, 2011 at 15:43
1

Just a cautionary note from Python in a Nutshell about bytes: Avoid using the bytes type as a function with an integer argument. In v2 this returns the integer converted to a (byte)string because bytes is an alias for str, while in v3 it returns a bytestring containing the given number of null characters. So, for example, instead of the v3 expression bytes(6), use the equivalent b'\x00'*6, which seamlessly works the same way in each version.

holdenweb
– holdenweb

2017年08月20日 10:09:48 +00:00
Commented Aug 20, 2017 at 10:09
5

Just a note, that if you are trying to convert binary data to a string, you'll most likely need to use something like byte_string.decode('latin-1') as utf-8 doesn't cover the entire range 0x00 to 0xFF (0-255), check out the python docs for more info.

iggy12345
– iggy12345

2019年07月10日 14:25:58 +00:00
Commented Jul 10, 2019 at 14:25

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How to convert string to bytes in Python 3

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