Python: How to define a function that gets a list of strings OR a string

Question

Let's say I want to add a list of strings or just one to a DB:

names = ['Alice', 'Bob', 'Zoe']

and I want that add_to_db will accept both cases

add_to_db(names)
add_to_db('John')

Is this the correct way to go:

def add_to_db(name_or_names):
    if (...):
        ... # add a single name
    else:
        for name in name_or_names:
            ... # add a single name
    commit_to_db()

What should I put in the first ..., as condition to whether it's a list of strings or just a string (don't forget that string is also iterable)?

You could also just pass 'John' as a single item list, e.g., ['John'], and define the function expecting the for — hexparrot
– hexparrot, Commented May 18, 2012 at 14:40
Yeah, but as I already wrote in another comment I have to support add_to_db('John') as this is already implemented and I don't want to change all calls to the functions — zenpoy
– zenpoy, Commented May 18, 2012 at 14:43

Kenan Banks · Accepted Answer · 2012-05-18 14:36:01Z

7

I think your best bet is to cast a single string to a 1-element list, and then have the rest of your function deal exclusively with lists.

def add_to_db(name_or_names):
    import types
    if isinstance(name_or_names, types.StringTypes):
        name_or_names = [name_or_names]
    try:
        for name in name_or_names:
            add_name(name)
        commit_to_db()
    except TypeError:
            # we didn't get a string OR a list >:(

answered May 18, 2012 at 14:36

Kenan Banks

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1 Comment

Ben Hoyt Over a year ago

Good approach, but two things: 1) you can just go isinstance(name_or_names, basestring), as the builtin basestring is a supertype of str and unicode; 2) your example will catch a TypeError raised by add_name() or commit_to_db(), which may hide bugs -- one should always try/except around the smallest amount of code possible. In this case, you'd usually just remove the try/except and let the error bubble up to the caller (it's a programming bug).

georg · Accepted Answer · 2012-05-18 14:27:55Z

4

Use keyword arguments:

def add_to_db(name=None, names=None):
    if name:
        .....
    elif names:
        ....
    else:
        raise ValueError("Invalid arguments")

add_to_db(name='John')
add_to_db(names=['explicit', 'is', 'better'])

answered May 18, 2012 at 14:27

georg

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1 Comment

zenpoy Over a year ago

That works as expected also with add_to_db('John') since it's the first arg. This is good for me because I'm extending an already existing function and I don't want to change all the old calls.

Álvaro H.G · Accepted Answer · 2021-10-14 15:39:09Z

2

You can declare the instance type like:

from typing import List, Union

def my_fancy_func(l: Union[List [str], str]):
  if not isinstance(l, list):
    print("Error", type(sentence))
  else:
    print("Everything's okay")

answered Oct 14, 2021 at 15:39

Álvaro H.G

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Comments

Ben Hoyt · Accepted Answer · 2012-05-18 14:36:30Z

1

It's possibly easier to test whether the object is a string rather than iterable. This is a technique I've used before. Make sure it's a list, then loop through it:

def add_to_db(name_or_names):
    if isinstance(name_or_names, basestring):
        # str or unicode, convert to list
        name_or_names = [name_or_names]
    for name in name_or_names:
        add_name_to_db()

answered May 18, 2012 at 14:36

Ben Hoyt

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Comments

Community · Accepted Answer · 2017-05-23 11:49:28Z

0

You can test if the argument has the __iter__ attribute, for which strings will fail:

if hasattr(arg, '__iter__'):
    print "I am an iterable that is not a string!"

From this answer.

edited May 23, 2017 at 11:49

CommunityBot

11 silver badge

answered May 18, 2012 at 14:25

matt b

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Comments

uselpa · Accepted Answer · 2012-05-18 14:36:09Z

-2

You can check the type of your parameter:

>>> type('John')
<type 'str'>
>>> type(['explicit', 'is', 'better'])
<type 'list'>

answered May 18, 2012 at 14:36

uselpa

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1 Comment

lvc Over a year ago

Checking for equality of types, as you seem to be suggesting (ie, test type(arg) == type('')) is almost always a bad idea. Doing isinstance(arg, str) will also match user-defined string subclasses, for example.

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