CODEWITHSUNDEEP
pythondatetime
danjjl
danjjl

Reputation: 462

Why is a datetime string format not reversible?

I expected datetime.strftime and datetime.strptime calls to be reversible. Such that calling 

datetime.strptime(datetime.now().strftime(fmt), fmt))

Would give the closest reconstruction of now() (given the information preserved by the format).

However, this is not the case when formatting a date to a string with a YYYY-Week# format:

>>> yyyy_u = datetime.datetime(1992, 5, 17).strftime('%Y-%U')
>>> print(yyyy_u)
'1992-20'

Formatting the string back to a date does not give the expected response:

>>> datetime.datetime.strptime(yyyy_u, '%Y-%U')
datetime.datetime(1992, 1, 1, 0, 0)

I would have expected the response to be the first day of week 20 in 1992 (17 May 1992).

Is this a failure of the %U format option or more generally are datetime.strftime and datetime.strptime calls not meant to be reversible?

Upvotes: 1

Views: 86

Answers (1)

Gagan T K
Gagan T K

Reputation: 728

From the Python docs regarding strptime() behaviour:

  1. When used with the strptime() method, %U and %W are only used in calculations when the day of the week and the year are specified.

Day of the week must be specified along with Week number and Year. (%Y-%U-%w)

datetime.datetime.strptime('1992-20-0', '%Y-%U-%w') gives the first day of week 20 for 1992 year.

Upvotes: 1

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