AttributeError: 'NoneType' object has no attribute 'replace'

AttributeError: 'NoneType' object has no attribute 'replace' #

The Python "AttributeError: 'NoneType' object has no attribute 'replace'" occurs when we try to call the replace() method on a None value, e.g. assignment from function that doesn't return anything. To solve the error, make sure to only call replace() on strings.

Here is a very simple example of how the error occurs.

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my_string = None

# ⛔️ AttributeError: 'NoneType' object has no attribute 'replace'
result = my_string.replace('z', 'a')


# ✅ if you need to check if not None before calling replace()
if my_string is not None:
    print('Variable is not None')
    print(my_string.replace('z', 'a'))
else:
    print('Variable is None')

Trying to call the replace() method on a None value is what causes the error.

The most common source of a None value (other than an explicit assignment) is a function that doesn't return anything.

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# 👇️ this function returns None
def get_string():
    print('zbc')


# 👇️ None
my_string = get_string()

# ⛔️ AttributeError: 'NoneType' object has no attribute 'replace'
result = my_string.replace('z', 'a')

Notice that our get_string function doesn't explicitly return a value, so it implicitly returns None.

The "AttributeError: 'NoneType' object has no attribute 'replace'" occurs for multiple reasons:

1. Having a function that doesn't return anything (returns None implicitly).
2. Explicitly setting a variable to None.
3. Assigning a variable to the result of calling a built-in function that doesn't return anything.
4. Having a function that only returns a value if certain condition is met.

If a variable might sometimes store a string and sometimes store None, you can explicitly check if the variable is not None before you call replace().

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my_string = 'one two four'

if my_string is not None:
    print('Variable is not None')
    result = my_string.replace('four', 'three')
    print(result)  # 👉️ "one two three"
else:
    print('Variable is None')

The if block will run only if the my_string variable does not store a None value, otherwise the else block runs.

Another common cause of the error is having a function that returns a value only if a condition is met.

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def get_string(a):
    if len(a) > 3:
        return a


my_string = get_string('hi')

print(my_string)  # 👉️ None

The if statement in the get_string function is only ran if the passed in string has a length greater than 3.

To solve the error in this scenario, you either have to check if the function didn't return None or return a default value if the condition is not met.

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def get_string(a):
    if len(a) > 3:
        return a
    return ''


my_string = get_string('hi')

print(my_string)  # 👉️ ""

Now the function is guaranteed to return a string regardless if the condition is met.