Why does python's datetime.datetime.strptime('201412', '%Y%m%d') not raise a ValueError?

Question

In the format I am given, the date 2014-01-02 would be represented by "20140102". This is correctly parsed with the standard strptime:

>>> datetime.datetime.strptime("20140102", "%Y%m%d")
datetime.datetime(2014, 1, 2, 0, 0)

In this format, "201412" would not be a valid date. The docs say that the "%m" directive is "Month as a zero-padded decimal number." It gives as examples "01, 02, ..., 12". The days directive "%d" is also supposed to be zero-padded.

Based on this, I expected that "201412" would be an invalid input with this format, so would raise a ValueError. Instead, it is interpreted as 2014-01-02:

>>> datetime.datetime.strptime("201412", "%Y%m%d")
datetime.datetime(2014, 1, 2, 0, 0)

The question is: is there a way to specify "no seriously zero-padded only"? Or am I misunderstanding the term "zero-padded" in this context?

Note that the question is not about how to parse dates in this format, but about understanding the behavior of strptime.

Answer 1

According to the related issue on the Python tracker, with the example being like such (a bit of a modification to this question, however the concept is the exact same):

>>> datetime.datetime.strptime('20141110', '%Y%m%d').isoformat()
'2014-11-10T00:00:00'
>>> datetime.datetime.strptime('20141110', '%Y%m%d%H%M').isoformat()
'2014-01-01T01:00:00'

The above behavior is determined to be not a bug as explained by this comment which states that they conform to the OpenGroup strptime standard which specifies that "leading zeros are permitted but not required.".

I guess the workaround is to use regex or check that the length of the string is of length 8 before passing into strptime.

Answer 2

If you look here at how the regex is defined for %m https://github.com/python/cpython/blob/2d264235f6e066611b412f7c2e1603866e0f7f1b/Lib/_strptime.py#L204

'm': r"(?P<m>1[0-2]|0[1-9]|[1-9])"

You can see you can either have a 10-12, 01-09, or 1-9 as acceptable months.