[Python-ideas] Extending __format__ method in ipaddress

[Eric Osborne]

Folks-

  The ipaddress library returns an IP address object which can represent
itself in a number of ways:


In [1]: import ipaddress

In [2]: v4 = ipaddress.IPv4Address('1.2.3.4')

In [3]: print(v4)
1.2.3.4

In [4]: v4
Out[4]: IPv4Address('1.2.3.4')

In [6]: v4.packed
Out[6]: b'\x01\x02\x03\x04'

In [9]: str(v4)
Out[9]: '1.2.3.4'

In [10]: int(v4)
Out[10]: 16909060

In [13]: bin(int(v4))
Out[13]: '0b1000000100000001100000100'

In [14]: hex(int(v4))
Out[14]: '0x1020304'

In [15]: oct(int(v4))
Out[15]: '0o100401404'

There are IPv6 objects as well:

In [6]: v6 =
ipaddress.IPv6Address('2001:0db8:85a3:0000:0000:8a2e:0370:7334')

In [7]: int(v6)
Out[7]: 42540766452641154071740215577757643572

and what I'm proposing will work for both address families.
In either case, bin/hex/oct don't work on them directly, but on the integer
representation.  This is a little annoying but not such a big deal.  What
is a big deal (at least to me) is that the binary representation isn't
zero-padded.  This makes it harder to compare two IP addresses by eye to
see what the differences are, i.e.:

In [16]: a = ipaddress.IPv4Address('0.2.3.4')
In [30]: bin(int(a))
Out[30]: '0b100000001100000100'

In [31]: bin(int(v4))
Out[31]: '0b1000000100000001100000100'

  It would be nice if there was a way to have an IP address always present
itself in fully zero-padded binary (32 bits for IPv4, 128 bits for IPv6).
I find this particularly convenient when putting together  training
material, as it's easier to show subnetting and aggregation if you point at
the binary than if you give people dotted-quad addresses and ask them to do
the binary conversion in their head.  Hex is also handy when you're
comparing a dotted-quad IP address to a hex sniffer trace.  It's possible
to do this in a one-liner (thanks to Eric Smith): f'{int(v4):#0{34}b}'.  But
this is a little cryptic.

  I opened bpo-32820 (https://github.com/python/cpython/pull/5627) to
contribute a way to do this.  I started with an __index__ method but Issue
15559 (
https://github.com/python/cpython/commit/e0c3f5edc0f20cc28363258df501758c1bdb1ca7)
rules this out.  I instead added a bits() class method so that v4.bits
would return the fully padded string.  This was not terribly pretty, but it
mirrored packed(), at least.

  Nick Coghlan suggested I instead extend __format__, which is what the
diffs in the current pull request do.  This allows a great deal more
flexibility: the current code takes 'b', 'n', or 'x' types, as well as the
'#' option and support for the '_' separator.  I realize now I didn't add
'o' but I certainly can for completeness.  I debated adding rfc1924
encoding for IPv6 addresses but decided it was entirely too silly.

  This is just a convenience function, but IMO fills a need.  Is this worth
pursuing?





eric
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