Python all() function

Python all() function is one of the built-in functions. It takes iterable as an argument and returns True if all elements of the iterable are true or it’s empty.

Python all() function

Python all() function is a utility method and shortcut to below function.

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def all(iterable):
    for element in iterable:
        if not element:
            return False
    return True

Let’s look at some of the examples of python all() function.

Python all() example with boolean

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# iterable has all True
list_bools = [True, True, True]

print(all(list_bools))

# iterable all elements are not True
list_bools = [True, True, False]

print(all(list_bools))

Output:

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True
False

Python all() with empty iterable

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# iterable is empty
list_bools = []

print(all(list_bools))

Output:

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True

Python all() with list of strings

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# iterable elements are True string
list_strs = ['True', 'True']

print(all(list_strs))

# iterable all elements are true string with different case
list_strs = ['True', 'true']

print(all(list_strs))

# iterable all elements are not true string
list_strs = ['abc', 'true']

print(all(list_strs))

# iterable all elements are empty string
list_strs = ['', 'true']

print(all(list_strs))

Output:

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True
True
True
False

When we want an object boolean value, python looks for __bool__ function in the object.

If __bool__ function is not defined, then len() function is called if it’s defined. The object boolean value is considered as True if len() output is non-zero.

If a class defines neither __len__() nor __bool__() functions, all its instances are considered True.

Python all() with custom objects

Let’s test above explanation with a custom class. We will create a custom Person class and use its objects in the list and call all() function on it.

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class Person:
    name = ""

    def __init__(self, n):
        self.name = n

list_objs = [Person("Pankaj"), Person("Lisa")]
print(all(list_objs))

list_objs = [Person("A"), Person("David")]
print(all(list_objs))

Output:

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True
True

Since our object doesn’t have __len__() and __bool__() function defined, it’s boolean value is True.

Let’s go ahead and define __len__() function for the Person class as below.

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   def __len__(self):
        print('len function called')
        return len(self.name)

Now the output of earlier code snippets will be:

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len function called
len function called
True
len function called
len function called
True

Notice that len() function is getting called for each object when all() is used with the list of Person objects.

Now let’s define __bool__ function for the Person class and see what happens with the above code.

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    def __bool__(self):
        print('bool function called')
        if len(self.name) > 3:
            return True
        else:
            return False

Output:

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bool function called
bool function called
True
bool function called
False

It’s clear from the output that if __bool__ function is defined, then it’s used for getting the python object boolean value. Notice that second list all() function output is False because ‘A’ length is less than 3.

That’s all for python all() function examples.

Reference: Official Documentation

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