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

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

The Python "TypeError: object of type 'generator' has no len()" occurs when we pass a generator object to the len() function. To solve the error, convert the generator to a list before passing it to the len() function.

Here is an example of how the error occurs.

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def g():
    yield 1
    yield 2
    yield 3


gen = g()

print(type(gen))  # 👉️ <class 'generator'>

# ⛔️ TypeError: object of type 'generator' has no len()
print(len(gen))

We passed a generator object to the len() function which caused the error.

One way to solve the error is to convert the generator object to a list.

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def g():
    yield 1
    yield 2
    yield 3


# 👇️ convert to list
my_list = list(g())

print(my_list)  # 👉️ [1, 2, 3]

print(len(my_list))  # 👉️ 3

We passed the generator object to the list() class to convert it to a list.

Note that once the generator is converted to a list, it is exhausted.

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def g():
    yield 1
    yield 2
    yield 3


# 👇️ convert to list
gen = g()

print(list(gen))  # 👉️ [1, 2, 3]

print(list(gen))  # 👉️ []

This is why you have to convert the generator to a list and store the result in a variable.

You can iterate over a generator with a simple for loop.

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def g():
    yield 1
    yield 2
    yield 3


gen = g()

for i in gen:
    print(i)  # 👉️ 1, 2, 3

Alternatively, you can pass the generator to the next() function to retrieve the next item.

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def g():
    yield 1
    yield 2
    yield 3


gen = g()

print(next(gen))  # 👉️ 1
print(next(gen))  # 👉️ 2
print(next(gen))  # 👉️ 3

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 generator.

If you didn't expect the variable to store a generator object, 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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import types


def g():
    yield 1
    yield 2
    yield 3


# 👇️ convert to list
gen = g()

print(type(gen)) # 👉️ <class 'generator'>

# 👇️ True
print(isinstance(gen, types.GeneratorType))

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.

Conclusion #

The Python "TypeError: object of type 'generator' has no len()" occurs when we pass a generator object to the len() function. To solve the error, convert the generator to a list before passing it to the len() function.