`datetime.strftime` and `datetime.strptime` interprete %Y differently

Question

I use a statement as shown below to create a datetime object from a string:

t = datetime.strptime("0023-10-10", "%Y-%m-%d")

Later, somewhere in my code uses the t object and invoke the strftime method with the same format string:

t.strftime("%Y-%m-%d")

This causes a ValueError: year=23 is before 1900; the datetime strftime() methods require year >= 1900.

It seems that the validation of the %Y input is different in this two similar methods. So I have to do the following to make sure I don't accept some bad years like 23:

try:
    format = "%Y-%m-%d"
    t = datetime.strptime("0023-10-10", format)
    t.strftime(format)
except ValueError:
    ...

I wonder if there's a better way to do this validation.

Solution 1

I like your idea of using a try..except to validate the input, since in some future version of Python, years < 1000 might be acceptable.

This comment in the code suggests this restriction is limited to Python's current implementation of strftime.

---

In Python 2.7, the exception occurs for years < 1900, but in Python 3.2, the exception occurs for years < 1000:

import datetime as dt
format = "%Y-%m-%d"
t = dt.datetime.strptime("0023-10-10", format)
try:
    t.strftime(format)
except ValueError as err:
    print(err)

prints

year=23 is before 1000; the datetime strftime() methods require year >= 1000

Solution 2

You could simply check if t.year < 1900 and if it is return an error. No need to deliberately cause an exception.

Comments

[satoru]

Thx for replying. By the way, do you have any idea why these to method behave differently?

[satoru]

I still don't understand why this restriction is not applied to strptime.

[unutbu]

datetime.strftime ultimately calls some C function, strftime. The implementation used is restricted to years >= 1900, or 1000 depending on the version of Python. strptime is implemented in Python, and does not have the same restriction. See this feature request.