Python：在另一个函数中调用具有返回值的函数：错误处理？

Question

I have created the following Python module that converts 24hr time (00:00 to 23:59) into words. Function1() tests that the input string matches the correct 24hr format before trying to extract the 12hours, minutes and period (ie am = 'before midday' and pm = 'after midday'). The following works perfectly as a standalone function, but fails when I call it from function3() when the input string doesn't match the correct format and the try statement doens't execute therefore the return values are not returned. Function3() therefore fails as the following return values don't exist. How do I restructure function1() so that if the pattern doesn't match, the Python module stops running and only executes the else statement in function1() letting the end user know that the input string doesn't match the correct format?

"""
Created on 29 May 2017

Convert 24hr format time

into time as words

@author: PeterW
"""
# import site-packages and modules
import re
import time
import argparse


def function1(input_time):
    """convert time from 24hr format
    into 12hours, minutes, period"""
    period_type = {'AM': 'before midday', 'PM': 'after midday'}
    regexp_object = re.compile(r'^(([01]\d|2[0-3]):([0-5]\d)|23:59)$')
    if regexp_object.match(input_time):
        try:
            time_object = time.strptime(input_time, '%H:%M')
            suffix = time.strftime('%p', time_object)
            hours = int(time.strftime('%I', time_object))
            minutes = time_object[4]
            period = period_type.get(suffix)
            print("{0}:{1} {2}".format(hours, str(minutes).zfill(2), period))
            return hours, minutes, period
        except ValueError as err:
            print(err)
    else:
        print("{0} doesn't match required format 00:00 to 23:59"
              .format(input_time))


def function2():
    """create dictionary
    to lookup equivalent word
    of integer number"""
    words_lookup = {1: 'one', 2: 'two', 3: 'three', 4: 'four', 5: 'five',
                    6: 'six', 7: 'seven', 8: 'eight', 9: 'nine', 10: 'ten',
                    11: 'eleven', 12: 'twelve', 13: 'thirteen',
                    14: 'fourteen', 16: 'sixteen', 17: 'seventeen',
                    18: 'eighteen', 19: 'nineteen', 20: 'twenty',
                    21: 'twenty one', 22: 'twenty two', 23: 'twenty three',
                    24: 'twenty four', 25: 'twenty five', 26: 'twenty six',
                    27: 'twenty seven', 28: 'twenty eight', 29: 'twenty nine'}
    return words_lookup


def function3(input_time):
    """convert 24hr format time
    into time as words"""
    hours, minutes, period = function1(input_time)
    words_lookup = function2()
    if hours == 12:
        hours2 = words_lookup.get(1)
    else:
        hours2 = words_lookup.get(hours+1)
    if hours == 12 and minutes == 0 and period == 'before midday':
        time = 'Midnight'
    elif hours == 12 and minutes == 0 and period == 'after midday':
        time = 'Midday'
    elif minutes == 0:
        time = "{0} o'clock {1}.".format(str(words_lookup.get(hours)).title(),
                                         period)
    elif minutes == 15:
        time = "Quarter past {0} {1}.".format(words_lookup.get(hours),
                                              period)
    elif minutes == 30:
        time = "Half past {0} {1}.".format(words_lookup.get(hours),
                                           period)
    elif minutes == 45:
        time = "Quarter to {0} {1}.".format(hours2,
                                            period)
    elif minutes < 30:
        minutes = str(words_lookup.get(minutes)).capitalize()
        time = "{0} minutes past {1} {2}.".format(minutes,
                                                  words_lookup.get(hours),
                                                  period)
    else:
        minutes = str(words_lookup.get(60 - minutes)).capitalize()
        time = '{0} minutes to {1} {2}.'.format(minutes,
                                                hours2,
                                                period)
    print(time)


if __name__ == '__main__':
    parser = argparse.ArgumentParser(description="Convert 24hr format time into time as words")
    parser.add_argument('--input_time', metavar='string', required=True,
                        help='input time in following format 00:00 to 23:59')
    args = parser.parse_args()
    function3(input_time=args.input_time)

Final version below, thanks for the help.

"""
Created on 04 June 2017

Convert 24hr format time

into time as words

@author: PeterW
"""
# import site-packages and modules
import sys
import re
import time
import argparse


def validate_time_format(input_time):
    """Validate that input string
    matches require 24hr time format"""
    regexp_object = re.compile(r'^(([01]\d|2[0-3]):([0-5]\d)|23:59)$')
    if regexp_object.match(input_time):
        print("Processing: {0} into words".format(input_time))
        time_str = input_time
    else:
        print("{0} doesn't match required input format: 00:00 to 23:59".format(input_time))
        sys.exit()
    return time_str


def extract_time(time_str):
    """convert time from 24hr format
    into 12hours, minutes, period"""
    period_type = {'AM': 'before midday', 'PM': 'after midday'}
    try:
        time_object = time.strptime(time_str, '%H:%M')
        suffix = time.strftime('%p', time_object)
        hours = int(time.strftime('%I', time_object))
        minutes = time_object[4]
        period = period_type.get(suffix)
#         print("{0}:{1} {2}".format(hours, str(minutes).zfill(2), period))
        return hours, minutes, period
    except ValueError as err:
        print(err)


def time_lookup():
    """create dictionary
    to lookup equivalent word
    of integer number"""
    words_lookup = {1: 'one', 2: 'two', 3: 'three', 4: 'four', 5: 'five',
                    6: 'six', 7: 'seven', 8: 'eight', 9: 'nine', 10: 'ten',
                    11: 'eleven', 12: 'twelve', 13: 'thirteen',
                    14: 'fourteen', 16: 'sixteen', 17: 'seventeen',
                    18: 'eighteen', 19: 'nineteen', 20: 'twenty',
                    21: 'twenty one', 22: 'twenty two', 23: 'twenty three',
                    24: 'twenty four', 25: 'twenty five', 26: 'twenty six',
                    27: 'twenty seven', 28: 'twenty eight', 29: 'twenty nine'}
    return words_lookup


def time_as_words(input_time):
    """convert 24hr format time
    into time as words"""
    time_str = validate_time_format(input_time)
    hours, minutes, period = extract_time(time_str)
    words_lookup = time_lookup()
    if hours == 12:
        hours2 = words_lookup.get(1)
    else:
        hours2 = words_lookup.get(hours+1)
    if hours == 12 and minutes == 0 and period == 'before midday':
        time = 'Midnight'
    elif hours == 12 and minutes == 0 and period == 'after midday':
        time = 'Midday'
    elif minutes == 0:
        time = "{0} o'clock {1}.".format(str(words_lookup.get(hours)).title(),
                                         period)
    elif minutes == 15:
        time = "Quarter past {0} {1}.".format(words_lookup.get(hours),
                                              period)
    elif minutes == 30:
        time = "Half past {0} {1}.".format(words_lookup.get(hours),
                                           period)
    elif minutes == 45:
        time = "Quarter to {0} {1}.".format(hours2,
                                            period)
    elif minutes < 30:
        min_str = str(words_lookup.get(minutes)).capitalize()
        time = "{0} minute{1} past {2} {3}.".format(min_str,
                                                    "" if minutes == 1 else "s",
                                                    words_lookup.get(hours),
                                                    period)
    else:
        min_str = str(words_lookup.get(60 - minutes)).capitalize()
        time = '{0} minute{1} to {2} {3}.'.format(min_str,
                                                  "" if 60 - minutes == 1 else "s",
                                                  hours2,
                                                  period)
    print(time)


if __name__ == '__main__':
    parser = argparse.ArgumentParser(description="Convert 24hr format time into time as words")
    parser.add_argument("-t", "--input_time", metavar="time", type=str, required=True,
                        help='input time in the following format 00:00 to 23:59')
    args = parser.parse_args()
    time_as_words(input_time=args.input_time)

Answer 1

When your function1 fails, it implicitly returns a None . The caller has to be aware of this possibility:

retval = function1(input_time)
if retval is None:
    # do something - say, exit
else:
    hours, minutes, period = retval

Answer 2

Switch the if regexp_object.match(input_time) for an if not regexp_object.match(input_time) , and switch the logic around too:

def function1(input_time):
    """convert time from 24hr format
    into 12hours, minutes, period"""
    period_type = {'AM': 'before midday', 'PM': 'after midday'}
    regexp_object = re.compile(r'^(([01]\d|2[0-3]):([0-5]\d)|23:59)$')
    # Switch to checking if it isn't valid instead, and exit early if that's the case
    if not regexp_object.match(input_time):
        print("{0} doesn't match required format 00:00 to 23:59"
              .format(input_time))
        exit(-1)

    # Since we already returned if the regex didn't match, we know if it reaches this point it'll match
    try:
        time_object = time.strptime(input_time, '%H:%M')
        suffix = time.strftime('%p', time_object)
        hours = int(time.strftime('%I', time_object))
        minutes = time_object[4]
        period = period_type.get(suffix)
        print("{0}:{1} {2}".format(hours, str(minutes).zfill(2), period))
        return hours, minutes, period
    except ValueError as err:
        print(err)

Note that exit actually exits the program altogether. If you merely wanted to return something (such as None , to sign that something went wrong) to a further function, you could instead do return None and the function would return then and there.