How to accept only string type arguments in Python 3

Question

I am trying to create some argument parameters for my python file. I only want to accept String type as arguments. Below is my code of foo.py -

import argparse

if __name__ == "__main__":
    parser = argparse.ArgumentParser()

    # options to the user 
    parser.add_argument('--source_table', type = str, required = True)

    args = parser.parse_args()
    params = vars(args) # convert arguments to dictionary
    print(type(params['source_table'])) # prints <class 'str'>

When I give string arguments as -

>python foo.py --source_table=abc

It prints

<class 'str'>

whereas, even if I type

>python foo.py --source_table=123

it prints

<class 'str'>

I would like throw an error saying that only String type is accepted.

123 can be either a string or int though. What if it's a1? Is it still a valid string? If you want only alpha characters you might want to manually handle with str.isalpha(). — r.ook
– r.ook, Commented Mar 7, 2018 at 17:47
If you are only interested in some sort of a pattern you might need to use a regex to check the validity. — picmate 涅
– picmate 涅, Commented Mar 7, 2018 at 17:51
A link in support of what @Idlehands said: docs.python.org/3.4/library/stdtypes.html#string-methods — Mr. T
– Mr. T, Commented Mar 7, 2018 at 17:58

ryati · Accepted Answer · 2018-03-07 18:08:56Z

4

"123" is still a string, but it seems you want to use the alpha characters only. You could check the input before you continue the execution:

import argparse
import re # regex 
if __name__ == "__main__":
    parser = argparse.ArgumentParser()

    # options to the user 
    parser.add_argument('--source_table', type = str, required = True)

    args = parser.parse_args()
    params = vars(args) # convert arguments to dictionary
    src_table = params['source_table']
    # check for alpha only
    if re.match(r'^[a-zA-Z]+$', src_table) is None:
        print("bad input, alpha characters only")
        return
    # continue normal execution
    print(type(src_table)) # prints <class 'str'>

or you can make your own argparse type like here: Python argparse regex expression

edit @idlehands points out below that isalpha() will be sufficient enough for letters only. If you want to allow -, _ or other special characters, then regex is still the way to go. Update the regex in the above code to be re.match(r'^[a-zA-Z\-_]+$', src_table) to match - and _.

edited Mar 7, 2018 at 18:08

answered Mar 7, 2018 at 17:51

ryati

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4 Comments

r.ook Over a year ago

.isalpha() is sufficient for the test, no need to use regex for this particular use case...

ryati Over a year ago

Very good point. I forgot about isalpha(). However, if there are cases where something like test-new is allowed, then isalpha() will be False.

r.ook Over a year ago

test-new would also fail with your check, but yes, regex will give you more flexibility if you need it. I'd say OP's question is not exactly clear to determine whether .isalpha() is sufficient or a variant of regex is required.

hpaulj Over a year ago

This isalpha test could be put into a type function.

hpaulj · Accepted Answer · 2018-03-08 05:55:59Z

This requirement could be implemented with a type function:

def stringonly(astr):
    try:
        int(astr)
    except ValueError:
        return astr
    raise argparse.ArgumentTypeError('only a string type accepted')

This is sort of a ~int type. It raises an error only if the string can be interpreted as an integer. Other tests such as isalpha would be fine too. I could have raised aTypeError or ValueError, but ArgumentTypeError lets me customize the error message.

It would be used as:

In [22]: parser = argparse.ArgumentParser()
In [23]: parser.add_argument('-s','--source', type=stringonly);


In [24]: parser.parse_args('-s 123'.split())
usage: ipython3 [-h] [-s SOURCE]
ipython3: error: argument -s/--source: only a string type accepted
An exception has occurred, use %tb to see the full traceback.

SystemExit: 2

In [25]: parser.parse_args('-s foo'.split())
Out[25]: Namespace(source='foo')

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