TypeError: object of type 'bool' has no len() in Python

TypeError: object of type 'bool' has no len() in Python #

The Python "TypeError: object of type 'bool' has no len()" occurs when we pass a boolean value (True or False) to the len() function. To solve the error, make sure you aren't evaluating an expression in the call to the len() function.

Here is an example of how the error occurs.

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my_bool = True

# ⛔️ TypeError: object of type 'bool' has no len()
print(len(my_bool))

We passed a boolean (True or False) to the len() function which caused the error.

Make sure you aren't evaluating an expression in the call to the len() function.

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my_bool = True

# ⛔️ TypeError: object of type 'bool' has no len()
if len('hi' == 'hi'):
    print('success')

The expression 'hi' == 'hi' evaluates to True, so we end up passing a boolean to the len function.

Instead, you can call the len() function which each value.

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my_bool = True

if len('hi') == len('hi'):
    # 👇️ this runs
    print('success')

The len() function returns the length (the number of items) of an object.

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my_list = ['apple', 'banana', 'kiwi']

result = len(my_list)

print(result)  # 👉️ 3

Notice that the len() function cannot be called with a boolean.

If you didn't expect the variable to store a boolean value, you have to correct the assignment.

If you aren't sure what type a variable stores, use the built-in type() class.

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my_bool = False

print(type(my_bool))  # 👉️ <class 'bool'>
print(isinstance(my_bool, bool))  # 👉️ True

The type class returns the type of an object.

The isinstance function returns True if the passed in object is an instance or a subclass of the passed in class.

When we pass an object to the len() function, the object's __len__() method is called.

You can use the dir() function to print an object's attributes and look for the __len__ attribute.

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my_bool = True

print(dir(my_bool))

Or you can check using a try/except statement.

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my_bool = True

try:
    print(my_bool.__len__)
except AttributeError:
    # 👇️ this runs
    print('object has no attribute __len__')

We try to access the object's __len__ attribute in the try block and if an AttributeError is raised, we know the object doesn't have a __len__ attribute and cannot be passed to the len() function.

Conclusion #

The Python "TypeError: object of type 'bool' has no len()" occurs when we pass a boolean value (True or False) to the len() function. To solve the error, make sure you aren't evaluating an expression in the call to the len() function.