[python] Best way to convert string to bytes in Python 3?

There appear to be two different ways to convert a string to bytes, as seen in the answers to TypeError: 'str' does not support the buffer interface

Which of these methods would be better or more Pythonic? Or is it just a matter of personal preference?

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

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

The absolutely best way is neither of the 2, but the 3rd. The first parameter to encode defaults to 'utf-8' ever since Python 3.0. Thus the best way is

b = mystring.encode()

This will also be faster, because the default argument results not in the string "utf-8" in the C code, but NULL, which is much faster to check!

Here be some timings:

In [1]: %timeit -r 10 'abc'.encode('utf-8')
The slowest run took 38.07 times longer than the fastest. 
This could mean that an intermediate result is being cached.
10000000 loops, best of 10: 183 ns per loop

In [2]: %timeit -r 10 'abc'.encode()
The slowest run took 27.34 times longer than the fastest. 
This could mean that an intermediate result is being cached.
10000000 loops, best of 10: 137 ns per loop

Despite the warning the times were very stable after repeated runs - the deviation was just ~2 per cent.

Using encode() without an argument is not Python 2 compatible, as in Python 2 the default character encoding is ASCII.

>>> 'äöä'.encode()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 0: ordinal not in range(128)

It's easier than it is thought:

my_str = "hello world"
my_str_as_bytes = str.encode(my_str)
type(my_str_as_bytes) # ensure it is byte representation
my_decoded_str = my_str_as_bytes.decode()
type(my_decoded_str) # ensure it is string representation

Answer for a slightly different problem:

You have a sequence of raw unicode that was saved into a str variable:

s_str: str = "\x00\x01\x00\xc0\x01\x00\x00\x00\x04"

You need to be able to get the byte literal of that unicode (for struct.unpack(), etc.)

s_bytes: bytes = b'\x00\x01\x00\xc0\x01\x00\x00\x00\x04'

Solution:

s_new: bytes = bytes(s, encoding="raw_unicode_escape")

Reference (scroll up for standard encodings):

Python Specific Encodings