如果您不知道日期输入的格式，如何格式化日期字符串？

Question

I have to read through a ton of files based on their file name. They can vary from the format YMD, YMD, M_D_Y, or Y_M_D. There could be others but at the moment this is all I am given to work with.

I need to be able to extract the dates, which I have already done using a regular expression, and format them into the form YMD. For example if my input string is 06_12_2018, I need to be able to format that into 20180612 so I can do comparisons with another file later.

What I have tried so far:

def cleanDate(date):
    datePatterns = [“%Y%m%d”, “%Y_%m_%d”, “%Y-%m-%d”, “%m_%d_%Y”]
    for pattern in datePatterns:
        if date in datePatterns:
            return datetime.strftime(date, “%Y%m%d”)
        else:
            print “String format not found!”
            return

Now that I am looking at it, it does not make sense to do if date in datePatterns . What is the best way to approach this?

Answer 1

The best way will be to use try/except:

for pattern in datePatterns:
    try:
        return datetime.strptime(date, pattern)
    except ValueError:
        pass
else:
    # none of the datePatterns worked
    raise Exception('failed to parse')

Note that it is strptime you want here, not strftime . Reminder for the wetware: p is for p arsing, f is for f ormatting.

They can vary from the format YMD, YMD, M_D_Y, or Y_M_D. There could be others but at the moment this is all I am given to work with.

If there could be other formats, consider to use dateutil.parser instead, which uses heuristics to guess the format. It's fairly popular, battle-tested and reliable.

>>> from dateutil.parser import parse  # pip install python-dateutil
>>> parse("2018-05-12")
datetime.datetime(2018, 5, 12, 0, 0)