You can do all sorts of things with the calendar
module:
import calendar
year = int(raw_input('Enter year: '))
month = int(raw_input('Enter month number: '))
print(calendar.monthrange(year, month)[1])
This considers leap years just fine, too.
Example:
Enter year: 2012
Enter month number: 2
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solved Python amount of days in a month with a given year and month? [closed]