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@Lukasa Lukasa/
Last active Aug 29, 2015

What would you like to do?
A prototype Python header mapping.
import collections
class HTTPHeaderMap(collections.MutableMapping):
A structure that contains HTTP headers.
HTTP headers are a curious beast. At the surface level they look roughly
like a name-value set, but in practice they have many variations that
make them tricky:
- duplicate keys are allowed
- keys are compared case-insensitively
- duplicate keys are isomorphic to comma-separated values, *except when
they aren't*!
- they logically contain a form of ordering
This data structure is an attempt to preserve all of that information
while being as user-friendly as possible.
def __init__(self, *args, **kwargs):
# The meat of the structure. In practice, headers are an ordered list
# of tuples. This early version of the data structure simply uses this
# directly under the covers.
self._nvps = []
for arg in args:
for header in kwargs.items():
def __getitem__(self, key):
Unlike the dict __getitem__, this returns a list of items in the order
they were added.
values = []
for k, v in self._nvps:
if _keys_equal(k, key):
if not values:
raise KeyError()
return values
def __setitem__(self, key, value):
Unlike the dict __setitem__, this appends to the list of NVPs. It also
splits out headers that can be split on the comma.
self._nvps.extend(canonical_form((key, sub_val)))
def __delitem__(self, key):
Sadly, __delitem__ is kind of stupid here, but the best we can do is
delete all headers with a given key. To correctly achieve the 'KeyError
on missing key' logic from dictionaries, we need to do this slowly.
indices = []
for (i, (k, v)) in enumerate(self._nvps):
if _keys_equal(k, key):
if not indices:
raise KeyError()
for i in indices[::-1]:
def __iter__(self):
This mapping iterates like the list of tuples it is.
for pair in self._nvps:
yield pair
def __len__(self):
The length of this mapping is the number of individual headers.
return len(self._nvps)
def __contains__(self, key):
If any header is present with this key, returns True.
return any(_keys_equal(key, k) for k, _ in self._nvps)
def keys(self):
Returns an iterable of the header keys in the mapping. This explicitly
does not filter duplicates, ensuring that it's the same length as
for n, _ in self._nvps:
yield n
def items(self):
This mapping iterates like the list of tuples it is.
for item in self:
yield item
def values(self):
This is an almost nonsensical query on a header dictionary, but we
satisfy it in the exact same way we satisfy 'keys'.
for _, v in self._nvps:
yield v
def get(self, name, default=None):
Unlike the dict get, this returns a list of items in the order
they were added.
return self[name]
except KeyError:
return default
def __eq__(self, other):
return self._nvps == other._nvps
def __ne__(self, other):
return self._nvps != other._nvps
def canonical_form(k, v):
Returns an iterable of key-value-pairs corresponding to the header in
canonical form. This means that the header is split on commas unless for
any reason it's a super-special snowflake (I'm looking at you Set-Cookie).
SPECIAL_SNOWFLAKES = set('set-cookie', 'set-cookie2')
k = k.lower()
yield k, v
for sub_val in v.split(','):
yield k, sub_val
def _keys_equal(x, y):
Returns 'True' if the two keys are equal by the laws of HTTP headers.
return x.lower() == y.lower():

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shazow commented Mar 5, 2015

nvps is a weird variable name, I couldn't figure out what it meant. I guess I'm more used to the term key-value pairs than name-value pairs.

Maybe call it items? Seems like the most canonical Python parlance.

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