TypeError: isinstance() arg 2 must be a type, tuple of types, or a union

# TypeError: isinstance() arg 2 must be a type, tuple of types, or a union

The "TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union" occurs for 2 main reasons:

1. Passing a list instead of a tuple when checking if an object is a subclass of one of multiple classes.
2. Shadowing a built-in class by declaring a variable with the same name, e.g. list.

# Passing a list instead of a tuple to isinstance()

Here is an example of how the error occurs.

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a_list = [str, int, bool]

# ⛔️ TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union
result = isinstance('bobbyhadz.com', a_list)

We passed a list as the second argument to the isinstance() function which caused the error.

The isinstance function can either be passed a class or a tuple of classes.

# Pass a tuple to the isinstance() function

You can use the tuple() class to convert the list to a tuple if you are trying to check if an object is an instance of one of multiple classes.

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a_list = [str, int, bool]

result = isinstance('bobbyhadz.com', tuple(a_list))
print(result)  # 👉️ True

The isinstance() function returns True if the passed-in object is an instance or a subclass of the passed-in class or at least one of the classes in a tuple.

# Shadowing built-in classes

The error is also caused if you shadow a built-in class by declaring a variable with the same name.

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# 👇️ This shadows the built-in list class
list = ['bobby', 'hadz', 'com']

numbers = [1, 2, 3]

# ⛔️ TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union
result = isinstance(numbers, list)

We declared a variable named list which shadows the built-in list class.

# Rename the variable to solve the error

To solve the error, rename the variable in your code.

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a_list = ['bobby', 'hadz', 'com']

numbers = [1, 2, 3]

result = isinstance(numbers, list)
print(result)  # 👉️ True

We renamed the variable to a_list, so it no longer shadows the built-in list class.

# Use the type() class to get around the error

If you aren't able to rename the variable, use the type() class.

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list = ['bobby', 'hadz', 'com']

numbers = [1, 2, 3]

result = isinstance(numbers, type(list))
print(result)  # 👉️ True

The type class returns the type of an object.

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print(type(['bobby', 'hadz', 'com']))  # 👉️ <class 'list'>
print(type('bobbyhadz.com'))  # 👉️ <class 'str'>

Most commonly the return value is the same as accessing the __class__ attribute on the object.

You can also pass an object of the correct type to the type() class, it doesn't have to be the variable that shadows the built-in class.

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list = ['bobby', 'hadz', 'com']

numbers = [1, 2, 3]

result = isinstance(numbers, type([]))
print(result)  # 👉️ True

However, the best solution is to rename the variable in your code to something that doesn't shadow a built-in Python class.

# Conclusion

To solve the "TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union":

1. Make sure to only pass a class or a tuple of classes as the second argument. to the isinstance() function.
2. Make sure to not shadow built-in classes by declaring a variable with the same name.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Creating class instances from a Dictionary in Python
• How to get the File path of a Class in Python
• Creating class instances from a Dictionary in Python
• How to Create an incremental ID in a Class in Python
• Convert a string to a Class object in Python
• Purpose of 'return self' from a class method in Python
• Call a class method from another Class in Python
• How to access Parent class Attributes in Python
• Error: filename.whl is not supported wheel on this platform