_offset_map appears to have redundant data

[cancan101]

Many / most of the entries in _offset_map look like:

_offset_map['X'] = OffsetX()

where X is a string literal for what would have been returned by OffsetX().rule_code

See for example: https://github.com/pydata/pandas/blob/master/pandas/tseries/frequencies.py#L132

This makes adding to and maintaining the list of Offsets more cumbersome.

Manually populating _offset_map could be reserved for cases in which the entry in _offset_map is not the same as the rule_code. The remaining cases would be auto-populated using a classmethod on the Offset to return all possible Offsets.

A related map is _offset_names

One suggestion would be to change get_offset_name to return offset.rule_code in the else case rather than raising an exception. This would remove the need for _offset_names

[cancan101]

I was able to verify that ALL entries in _offset_map have the form _offset_map[OffsetX().rule_code] = OffsetX():

    def test_offset_map(self):
        for name,offset in pandas.tseries.frequencies._offset_map.iteritems():
            assert name == None if offset is None else offset.rule_code

Given that is there any reason to keep the code looking like this:

_offset_map = {
    'D': Day(),
    'C': cday,
    'B': BDay(),
    'H': Hour(),
    'T': Minute(),
    'S': Second(),
    'L': Milli(),
    'U': Micro(),
...

[cpcloud]

Instead of assert name == None do assert name is None because there is only ever a single object with the value None (i.e., you can't make distinct None objects like you can with almost every other kind of object). It's also standard practice when writing Python.

[cancan101]

@cpcloud I actually think you are wrong there :-)

[cancan101]

On second look, my precedence is wrong.

[cancan101]

The == was intentional with the None as it was supposed to say:

assert name == (None if offset is None else offset.rule_code)

[cpcloud]

@cancan101 Not a big deal. You can also use a plain ol' if..else suite here; no need to scrunch things up if you don't need to.

[cancan101]

That is what happens when you write the code initially without any if/else and then realize there are Nones in there.

[cancan101]

Also now that the test is fixed, it DOES fail... Looking into that.

[cancan101]

Okay. So it looks like that test will fail with 'QS', 'BQS', 'BQ', not surprisingly since they are constructed like:

'QS': QuarterBegin(startingMonth=1),
'QS-JAN': QuarterBegin(startingMonth=1),

Any reason not to just put the 'QS' et. al. entries into the _rule_aliases?

As a side note:

This code is not well defined for the above situations:

_offset_names = {}
for name, offset in compat.iteritems(_offset_map):
    if offset is None:
        continue
    offset.name = name
    _offset_names[offset] = name

[cancan101]

#5148 is my WIP for this. So far the above unit test passes and the values in _offset_names are no longer ambiguous.