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# Coding Anti-Patterns
This document contains some anti-patterns that have either been discussed during code-reviews or noticed by some one when refactoring any of GN's code-base. Use these "ideas" as a reference, and apply reasonable judgement depending on what you are working on.
## [Python] Don't pass mutable types as arguments
Consider:
```
from typing import List
_l = ['a', 'b', 'c']
def add_pvalue(l: List, val: str) -> List:
l.append(val)
return l
print(f"{_l=}\n")
print(f"{add_pvalue(_l, 'd')=}\n")
print(f"{_l=}")
```
which outputs (note that _l in the global scope has changed):
```
_l=['a', 'b', 'c']
add_pvalue(_l, 'd')=['a', 'b', 'c', 'd']
_l=['a', 'b', 'c', 'd']
```
A better fix would be:
```
from typing import List
_l = ['a', 'b', 'c']
def add_pvalue(l: List, val: str) -> List:
l = l.copy()
l.append(val)
return l
print(f"{_l=}\n")
print(f"{add_pvalue(_l, 'd')=}\n")
print(f"{_l=}")
```
which now does the right thing:
```
_l=['a', 'b', 'c']
add_pvalue(_l, 'd')=['a', 'b', 'c', 'd']
_l=['a', 'b', 'c']
```
Best, use immutable types:
```
from typing import Tuple
_l = ['a', 'b', 'c']
def add_pvalue(l: Tuple, val: str) -> Tuple:
return l + (val,)
print(f"{_l=}\n")
print(f"{add_pvalue(tuple(_l), 'p')=}\n")
print(f"{_l=}")
```
which outputs:
```
_l=['a', 'b', 'c']
add_pvalue(tuple(_l), 'p')=('a', 'b', 'c', 'p')
_l=['a', 'b', 'c']
```
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