Returning True or False vs. True or None

[DeaD_EyE]

In some cases, not always, functions return something and if an error happens or there was no match, they return None explicit or implicit.

Explicit None

def foo():
    return None

Implicit None

def foo():
    pass

The implicit None returns always, if no other return-statement was executed.


If you get from a function some type back and you have to check for None, just recognize that True, False, None are singletons and they're unique. If a bad function, not your code, returns sometimes a list with strings, sometimes None and sometimes an empty list, then you have to do two checks.

First you check, if the return type is a None.
If this check passes, you check if the list is not empty.

import random


def silly_function():
    return_vals = (None, [], ['Hello', 'World'])
    return random.choice(return_vals)

# now the tests are more complex, if you
# want to handle the None explicit

return_value = silly_function()
if return_value is None:
    print('None')
elif return_value:
    print('The list is not empty')
else:
    # no None, but a empty list
    print('The list is empty')

Sometimes we have to handle this.
A better approach is this:

def find(pattern):
    words = ('foo', 'bar', 'baz', 'fizz', 'fuzz')
    results = []
    for word in words:
        if word.startswith(pattern):
            results.append(word)
            # MATCH
    return results

# the return type of results is always a list
# which is good, because lesser cases to check

return_value = find('f')
if return_value:
    print(return_value)
else:
    print('Sorry, I did not find any matches. The list is empty')

The line if return_value: does an implicit bool(return_value).
If the list is empty, bool(your_list) returns False.

So in general the boolean of sequences and containers is False, if the container or sequence is empty.
Otherwise it's True.

True and False are subtypes of int.
This expression is True: (0.0 == 0 == False)
This expression is True: (1.0 == 1 == True)
Other funny stuff: (True + True == 2)

Counting True in a list, where only True and False are inside.

true_false_list = [True, False, True, True, False, True]
# sum True
trues = sum(true_false_list)
print('True is', trues, 'times in the list')

# of course you can use also
# true_false_list.count(True)

All values of integers and floats are evaluated as boolean to True, if the value is not zero.

values = (True, False, None, 0, -1, 1, 10, 0.0, 5.5, -5.5, [], (1,2,3), [1])
# a tuple with some values
for value in values:
    print(f'The boolean of the value {value!s:<10} is {bool(value)}')

Output:
The boolean of the value True       is True
The boolean of the value False      is False
The boolean of the value None       is False
The boolean of the value 0          is False
The boolean of the value -1         is True
The boolean of the value 1          is True
The boolean of the value 10         is True
The boolean of the value 0.0        is False
The boolean of the value 5.5        is True
The boolean of the value -5.5       is True
The boolean of the value []         is False
The boolean of the value (1, 2, 3)  is True
The boolean of the value [1]        is True

With this knowledge you can simplify your code and write it more pythonic.