# Python mutable default arguments: why your list keeps growing (tested examples)

A list that keeps growing between function calls is one of the most common Python surprises in exams and interviews. Here is why it happens, with output I ran on Python 3.10.

## The bug

```python
def add_bad(x, bucket=[]):
    bucket.append(x)
    return bucket

print(add_bad(1))
print(add_bad(2))
print(add_bad(3))
```

Output:

```
[1]
[1, 2]
[1, 2, 3]
```

You expected `[1]`, `[2]`, `[3]`. Python builds the default value once, when `def` runs, not on each call. Every call that skips `bucket` reuses the same list. You can see it:

```python
print(add_bad.__defaults__)   # ([1, 2, 3],)
```

## The fix

Use `None` as the default and create the list inside the function.

```python
def add_ok(x, bucket=None):
    if bucket is None:
        bucket = []
    bucket.append(x)
    return bucket

print(add_ok(1))   # [1]
print(add_ok(2))   # [2]
```

## Same family: shallow copies

`list.copy()` copies only the outer list. The inner lists are shared.

```python
import copy
a = [[1, 2], [3, 4]]
b = a.copy()
b[0].append(99)
print(a)             # [[1, 2, 99], [3, 4]]  a changed too

c = copy.deepcopy(a)
c[0].append(5)
print(a, c)          # a is unchanged by c
```

## Same family: the grid trap

```python
grid = [[0] * 3] * 2
grid[0][0] = 7
print(grid)    # [[7, 0, 0], [7, 0, 0]]  both rows are one list

grid2 = [[0] * 3 for _ in range(2)]
grid2[0][0] = 7
print(grid2)   # [[7, 0, 0], [0, 0, 0]]
```

`[x] * n` repeats a reference to the same object. A list comprehension builds a new row each time.

## Quick rule

- Never use a list, dict or set as a default argument. Use `None`.
- `copy()` and slicing are shallow. Use `copy.deepcopy` for nested data.
- Build 2D grids with a comprehension, not `*`.

Try to predict the output of each snippet before you run it. That habit is what exam questions test.

I wrote a longer version with 7 more tested examples and exam-style questions here: [Python Mutable Default Arguments and Shallow Copies](https://techvaniai.blogspot.com/2026/10/python-mutable-default-arguments-shallow-copy.html). It is part of TechVani, a free BTech CSE notes and lab-files site.