Parse string and assign to the same variable?

Question

Is it an pythonic way to re-assign variables with values of different types?

I have a string of the form k1:v1,k2:v2,k3:v3, etc which comes from the command line. It is straightforward to parse it:

kvs=args.kvs
if kvs is not None:
    kvs = [(kv[0], kv[1]) for kv in [kv_str.split(':') for kv_str in kvs.split(',')]]

Since I came to python from strictly-typed languages the approach does not look really clean to me since it is re-assigned with a different type.

Would it be considered a pythonic solution of parsing the string?

Yes, it is usually fine on a case-by-case basis. I would consider this OK. This is really an opinion-based question though. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Apr 29, 2020 at 21:32
FWIW you can simplify: kvs = [tuple(kv.split(':')) for kv in kvs.split(',')] — wjandrea
– wjandrea, Commented Apr 29, 2020 at 21:34
None of the v values can contain a :, right? If they can, use split(':', 1). — chepner
– chepner, Commented Apr 29, 2020 at 21:35
If you are using argparse, you might consider using a custom type to unpack the string during parsing, rather than after. p.add_argument('--kvs', type=lambda kvs: [tuple(kv.split(':', 1)) for kv in kvs.split(",")]). — chepner
– chepner, Commented Apr 29, 2020 at 21:42

Kevin Welch · Accepted Answer · 2020-04-30 20:51:08Z

5

Short Answer:

That is fine a way to do it. Although you can just do if kvs: since empty collection types are considered falsey in Python (if that is appropriate for your case of course).

Long Answer:

This is really just opinion in Python world unfortunately. When coming from strictly-typed languages to Python you will see people do things that are discouraged or prohibited in a stricter language.

That being said I think it is always safer to avoid mutation whenever possible and to create a new variable if the type changes. In large codebases where args are passed around with no type checking, this approach can save you a lot of time debugging in the future.

Another way you may see this written is something like this that assigns the list object to a new variable.

kvs = "k1:v1,k2:v2,k3:v3"

kv_pairs = [
    tuple(kv_str.split(':'))
    for kv_str in kvs.split(',')
] if kvs else []

EDIT: fixed the code sample I posted

edited Apr 30, 2020 at 20:51

answered Apr 29, 2020 at 21:39

Kevin Welch

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4 Comments

wjandrea Over a year ago

"you can just do if kvs: though" - I definitely wouldn't do that, in case you want to pass an empty string: --kvs '' -> kvs = [] instead of kvs = None. If --kvs is not provided, then kvs = None.

Kevin Welch Over a year ago

Empty string/array/dict/set and None all are falsey. Try it out. Set kvs to None or some different empty data structures to and it will result in kv_pairs == []. If you wanted to do type checking before the comprehension was run for some reason, it would be better to explicitly raise a TypeError.

wjandrea Over a year ago

I mean OP is talking about command line argument handling, and those are always strings. As well OP's code leaves kvs == None if it's already None. I just updated my answer to show what I mean. No flag -> kvs = None, with flag and empty string -> kvs = []

Kevin Welch Over a year ago

Oh I see. Thank you for clarifying!

wjandrea · Accepted Answer · 2020-04-29 22:03:02Z

2

It's not bad, but you might find it cleaner to parse it during argument parsing. For example in argparse, you can use a custom type:

import argparse

def kvs_string(kvs):
    if not kvs:
        return []  # Since str.split will return "['']"
    return [tuple(kv.split(':')) for kv in kvs.split(",")]

parser = argparse.ArgumentParser()
parser.add_argument('--kvs', type=kvs_string)

for args in [], ['--kvs', ''], ['--kvs', 'k1:v1,k2:v2,k3:v3']:
    print(parser.parse_args(args))

Output:

Namespace(kvs=None)
Namespace(kvs=[])
Namespace(kvs=[('k1', 'v1'), ('k2', 'v2'), ('k3', 'v3')])

edited Apr 29, 2020 at 22:03

answered Apr 29, 2020 at 21:49

wjandrea

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