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.

Copy
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

Copy
# 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:

Copy
True
False

Python all() with empty iterable

Copy
# iterable is empty
list_bools = []

print(all(list_bools))

Output:

Copy
True

Python all() with list of strings

Copy
# 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:

Copy
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.

Copy
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:

Copy
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.

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

Now the output of earlier code snippets will be:

Copy
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.

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

Output:

Copy
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

About Pankaj

If you have come this far, it means that you liked what you are reading. Why not reach little more and connect with me directly on Google Plus, Facebook or Twitter. I would love to hear your thoughts and opinions on my articles directly.

Recently I started creating video tutorials too, so do check out my videos on Youtube.