constify

Constify

The constify library provides the freezeparams decorator to make mutable default values and arguments passed to functions immutable.

Why use the freezeparams decorator ?

[1] The default value of a parameter is evaluated only once, which can lead to unexpected behavior if the value is a mutable object, such as a list or a dictionary. For example, the following function accumulates arguments across calls:

Example

def f(a, L=[]):
    L.append(a)
    return L

print(f(1))  # [1]
print(f(2))  # [1, 2]
print(f(3))  # [1, 2, 3]

To prevent this default value from being shared, the function can be rewritten as follows:

def f(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L

This solution is effective, but with multiple mutable default values, it requires many checks. The freezeparams decorator simplifies the use of mutable objects as default values and arguments.

Installation

pip install constify

Usage

from constify import freezeparams

@freezeparams
def f(a, L=[]):
    L.append(a)
    return L

# Default value (list) remains unchanged
print("f(1) =>", f(1))  # [1]
print("f(2) =>", f(2))  # [2]
print("f(3) =>", f(3))  # [3]

print("-" * 25) # just a separator

# Passed list remains intact
x = [1, 2]
print("x =", x)  # [1, 2]
print(f"f(3, {x}) =>", f(3, x))  # [1, 2, 3]
print(f"f(4, {x}) =>", f(4, x))  # [1, 2, 4]
print("x =", x)  # [1, 2]

You can comment out the decorator and run the code again to see the difference.

Simple and effective!

Sources

[1] https://docs.python.org/3.13/tutorial/controlflow.html#default-argument-values