Why can't Python's string.format pad with "\x00"?

Question

I wanted to pad a string with null characters ("\x00"). I know lots of ways to do this, so please do not answer with alternatives. What I want to know is: Why does Python's string.format() function not allow padding with nulls?

Test cases:

>>> "{0:\x01<10}".format("bbb")
'bbb\x01\x01\x01\x01\x01\x01\x01'

This shows that hex-escaped characters work in general.

>>> "{0:\x00<10}".format("bbb")
'bbb       '

But "\x00" gets turned into a space ("\x20").

>>> "{0:{1}<10}".format("bbb","\x00")
'bbb       '
>>> "{0:{1}<10}".format("bbb",chr(0))
'bbb       '

Even trying a couple other ways of doing it.

>>> "bbb" + "\x00" * 7
'bbb\x00\x00\x00\x00\x00\x00\x00'

This works, but doesn't use string.format

>>> spaces = "{0: <10}".format("bbb")
>>> nulls  = "{0:\x00<10}".format("bbb")
>>> spaces == nulls
True

Python is clearly substituting spaces (chr(0x20)) instead of nulls (chr(0x00)).

Answer 1

Digging into the source code for Python 2.7, I found that the issue is in this section from ./Objects/stringlib/formatter.h, lines 718-722 (in version 2.7.3):

/* Write into that space. First the padding. */
p = fill_padding(STRINGLIB_STR(result), len,
                 format->fill_char=='\0'?' ':format->fill_char,
                 lpad, rpad);

The trouble is that a zero/null character ('\0') is being used as a default when no padding character is specified. This is to enable this behavior:

>>> "{0:<10}".format("foo")
'foo       '

It may be possible to set format->fill_char = ' '; as the default in parse_internal_render_format_spec() at ./Objects/stringlib/formatter.h:186, but there's some bit about backwards compatibility that checks for '\0' later on. In any case, my curiosity is satisfied. I will accept someone else's answer if it has more history or a better explanation for why than this.