Dangerous default argument PYL-W0102
Bug risk
Critical
4 years ago
—
4 years old
Dangerous default value [] as argument
29
30
31class BroadcastRouter(DatapointRouter):
32 def __init__(self, destinations=[]): 33 self.destinations = set(destinations)
34
35 def addDestination(self, destination):
Description
Do not use a mutable like list or dictionary as a default value to an argument. Python’s default arguments are evaluated once when the function is defined. Using a mutable default argument and mutating it will mutate that object for all future calls to the function as well.
Bad practice
def my_function(elem, l=[]):
l.append(elem)
return l
print(my_function(2)) # [2]
print(my_function(5)) # [2, 5]
Recommended:
def my_function(elem, l=None):
if l is None:
l = []
l.append(elem)
return l
print(my_function(2)) # [2]
print(my_function(5)) # [5]