python - Why do I need 'b' to encode a string with Base64?

Question

Following this python example, I encode a string as Base64 with:

>>> import base64
>>> encoded = base64.b64encode(b'data to be encoded')
>>> encoded
b'ZGF0YSB0byBiZSBlbmNvZGVk'

But, if I leave out the leading b:

>>> encoded = base64.b64encode('data to be encoded')

I get the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:Python32libase64.py", line 56, in b64encode
   raise TypeError("expected bytes, not %s" % s.__class__.__name__)
   TypeError: expected bytes, not str

Why is this?

Answer 1

base64 encoding takes 8-bit binary byte data and encodes it uses only the characters A-Z, a-z, 0-9, +, /* so it can be transmitted over channels that do not preserve all 8-bits of data, such as email.

Hence, it wants a string of 8-bit bytes. You create those in Python 3 with the b'' syntax.

If you remove the b, it becomes a string. A string is a sequence of Unicode characters. base64 has no idea what to do with Unicode data, it's not 8-bit. It's not really any bits, in fact. :-)

In your second example:

>>> encoded = base64.b64encode('data to be encoded')

All the characters fit neatly into the ASCII character set, and base64 encoding is therefore actually a bit pointless. You can convert it to ascii instead, with

>>> encoded = 'data to be encoded'.encode('ascii')

Or simpler:

>>> encoded = b'data to be encoded'

Which would be the same thing in this case.

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* Most base64 flavours may also include a = at the end as padding. In addition, some base64 variants may use characters other than + and /. See the Variants summary table at Wikipedia for an overview.