Never use mutable objects (lists, dictionaries, sets) as default parameter values in function definitions. This creates a shared object across all function calls, leading to unexpected behavior where modifications persist between calls.
Never use mutable objects (lists, dictionaries, sets) as default parameter values in function definitions. This creates a shared object across all function calls, leading to unexpected behavior where modifications persist between calls.
Instead, use None as the default value and assign the mutable object inside the function body. This ensures each function call gets a fresh instance of the mutable object.
Problem:
def get_project(project_dir, config_path=None, enabled_profiles=[]):
# enabled_profiles is shared across all calls!
enabled_profiles.append("default")
return enabled_profiles
def project_from_options(project_dir, options, additional_options={}):
# additional_options is shared across all calls!
additional_options["key"] = "value"
return additional_options
Solution:
def get_project(project_dir, config_path=None, enabled_profiles=None):
enabled_profiles = enabled_profiles or []
enabled_profiles.append("default")
return enabled_profiles
def project_from_options(project_dir, options, additional_options=None):
additional_options = additional_options or {}
additional_options["key"] = "value"
return additional_options
This pattern prevents the “least astonishment” principle violation where the same mutable object is reused across function invocations, causing side effects that can be difficult to debug.
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