Why Python’s Mutable Default Arguments Cause Hidden Bugs (And How to Fix Them)

Understanding Python’s Mutable Default Arguments

Python beginners often encounter confusing bugs caused by using a mutable object as a function’s default value. This article explains why the default value is created only once at function definition, how it leads to unexpected results, and how to avoid the problem.

Consider a function that defines a list parameter with a default empty list and appends a number. When the function is called without providing the argument, the list retains the previously appended value because the same list object is reused each call.

The root cause is that Python evaluates default arguments when the function is defined, storing the object for all subsequent calls. If the default is mutable (e.g., a list), modifications persist across calls, while immutable types such as integers are rebound to new objects.

To prevent this, a common pattern is to use None as the default and create a new list inside the function if needed. This ensures each call works with a fresh object.

Another typical pitfall involves using time.time() as a default argument. Because the default is evaluated once, every call returns the same timestamp—the time when the function was defined—rather than the current time.

The behavior is identical in Python 2.x and 3.x; the only syntactic difference is that print must be called as a function in Python 3.