datetime.timezone methods require datetime object

[poke]

BPO	22183
Nosy	@abalkin, @bitdancer

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2014-08-11.14:48:54.903>
created_at = <Date 2014-08-11.14:17:13.372>
labels = ['invalid', 'type-bug', 'library']
title = 'datetime.timezone methods require datetime object'
updated_at = <Date 2014-08-11.14:48:54.901>
user = 'https://bugs.python.org/poke'

bugs.python.org fields:

activity = <Date 2014-08-11.14:48:54.901>
actor = 'belopolsky'
assignee = 'none'
closed = True
closed_date = <Date 2014-08-11.14:48:54.903>
closer = 'belopolsky'
components = ['Library (Lib)']
creation = <Date 2014-08-11.14:17:13.372>
creator = 'poke'
dependencies = []
files = []
hgrepos = []
issue_num = 22183
keywords = []
message_count = 3.0
messages = ['225188', '225189', '225190']
nosy_count = 3.0
nosy_names = ['belopolsky', 'r.david.murray', 'poke']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue22183'
versions = []

[poke]

I’ve noticed that the methods in datetime.timezone all require a datetime object (or explicitely None) as its parameter or they will raise an exception.

The datetime object however is never required for the implementation of the method, so it seems to me like an unnecessary requirement, given that timezone objects are completely independent of datetime objects.

For example timezone.utcoffset is implemented like this:

    def utcoffset(self, dt):
        if isinstance(dt, datetime) or dt is None:
            return self._offset
        raise TypeError("utcoffset() argument must be a datetime instance"
                        " or None")

I don’t really understand this requirement and if there isn’t an actual reason for it (if there is, it should be documented somewhere), I’d suggest to remove this requirement completely. For backwards compatibility, the parameter could simply default to None.

[bitdancer]

A timezone is a particular implementation of the general tzinfo API, and in the general case the datetime argument *is* required. This is already documented (ie: that timezone is a concrete implementation of tzinfo, and that the tzinfo API requires the datetime).

[abalkin]

As David explained, utcoffset() method signature is dictated by the base class. This is not a bug.