Is there a function in Python that returns a number when it is given the day of the week as an input (string)?

You can get the attribute from the calendar module:

>>> import calendar
>>> getattr(calendar, "monday".upper())
0

A simple way to do so is to create a dictionary:

d = {'Monday': 0, 'Tuesday': 1, 'Wednesday': 3, ...}

and then d['Monday'] would return 1. Wrap it into a function if you want.

If you don't want to use the calendar module and are only concerned with English names of weekdays:

def weekday_index(name):
    days = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"]

    name = name.title()
    if name in days:
        return days.index(name)
        
    return -1

I would suggest using the calendar module. This offers the advantage of being locale independent.

import calendar

def get_day_num(day):
    try:
        return calendar.day_name[:].index(day.title())
    except ValueError:
        pass
    return -1

print(get_day_num('tuesday'))

Output:

1

Note:

print(get_day_num('dienstag'))

...would produce the same result in a German locale

Create a dictionary of days:

import calendar
days = dict(zip(calendar.day_name, range(7)))

Look up the day:

>>> days["monday".title()]
0