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# 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]](#sources) 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
```python
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:
```python
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
```bash
pip install constify
```
## Usage
```python
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
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