HighFile-scopePython Plug & play

Avoid Mutable Objects as Default Arguments

Identify function definitions where a mutable object (e.g., list, dictionary, set) is used as a default argument. This can cause shared state across function calls, leading to unintended behavior. Suggest using `None` as a default and initializing the object inside the function.

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Why this matters

Using mutable objects (like lists or dictionaries) as default arguments can cause unexpected behavior. Since Python reuses the same object across multiple function calls, changes persist between calls, leading to bugs that are difficult to debug.

Bad vs. good

Side-by-side examples engineers can pattern-match during review.

Avoid

def append_to_list(value, my_list=[]):
    my_list.append(value)
    return my_list

Prefer

def append_to_list(value, my_list=None):
    if my_list is None:
        my_list = []
    my_list.append(value)
    return my_list

More snippets

Bad

def append_to_list(value, my_list=[]):
    my_list.append(value)
    return my_list

Good

def append_to_list(value, my_list=None):
    if my_list is None:
        my_list = []
    my_list.append(value)
    return my_list

Related rules

From the same buckets as this rule.

Critical
Avoid equality operators in loop termination conditions
Check if loops use equality operators (== or !=) in termination conditions. These can lead to infinite loops if the condition is never met exactly. Instead, use relational operators like < or > for safer loop termination.
error-handlingreadability-refactor
Critical
Avoid Returning Null in Non-Async Task Methods
Returning null from a Task or Task<T> method causes a NullReferenceException if awaited. Use Task.CompletedTask or Task.FromResult<T>(null) instead.
error-handlingreadability-refactor+1
Critical
Avoid Using Boolean Expressions in `except` Statements
Detect `except` clauses that use `or` or `and` to catch multiple exceptions. This does not work as intended. Recommend using a tuple of exceptions instead.
error-handlingstack-python

Avoid Mutable Objects as Default Arguments

Bad vs. good

Side-by-side examples engineers can pattern-match during review.

Avoid

def append_to_list(value, my_list=[]):
    my_list.append(value)
    return my_list

Prefer

def append_to_list(value, my_list=None):
    if my_list is None:
        my_list = []
    my_list.append(value)
    return my_list

More snippets

Bad

def append_to_list(value, my_list=[]):
    my_list.append(value)
    return my_list

Good

def append_to_list(value, my_list=None):
    if my_list is None:
        my_list = []
    my_list.append(value)
    return my_list

Related rules

From the same buckets as this rule.

Critical

Avoid equality operators in loop termination conditions

Check if loops use equality operators (== or !=) in termination conditions. These can lead to infinite loops if the condition is never met exactly. Instead, use relational operators like < or > for safer loop termination.

error-handlingreadability-refactor

Critical

Avoid Returning Null in Non-Async Task Methods

Returning null from a Task or Task<T> method causes a NullReferenceException if awaited. Use Task.CompletedTask or Task.FromResult<T>(null) instead.

error-handlingreadability-refactor+1

Critical

Avoid Using Boolean Expressions in `except` Statements

Detect `except` clauses that use `or` or `and` to catch multiple exceptions. This does not work as intended. Recommend using a tuple of exceptions instead.

error-handlingstack-python