Assigning to built-in function

Python has a number of built-in functions that are always accessible in the interpreter. Unless you have a special reason, you should neither overwrite these functions nor assign a value to a variable that has the same name as a built-in function. Overwriting a built-in might have undesired side effects or can cause runtime errors. Python developers usually use built-ins ‘as-is’. If their behaviour is changed, it can be very tricky to trace back the actual error.


In the code below, the list built-in is overwritten. This makes it impossible, to use list to define a variable as a list. As this is a very concise example, it is easy to spot what the problem is. However, if there are hundreds of lines between the assignment to list and the assignment to cars, it might become difficult to identify the problem.

# Overwriting built-in 'list' by assigning values to a variable called 'list'
list = [1, 2, 3]
# Defining a list 'cars', will now raise an error
cars = list()
# Error: TypeError: 'list' object is not callable

Best practice

Unless you have a very specific reason to use variable names that have the same name as built-in functions, it is recommended to use a variable name that does not interfere with built-in function names.

# Numbers used as variable name instead of 'list'
numbers = [1, 2, 3]
# Defining 'cars' as list, will work just fine
cars = list()