Note: examples are coded in Python 2.x, but the basic point of the post applies to all versions of Python.
There’s a Python gotcha that bites everybody as they learn Python. In fact, I think it was Tim Peters who suggested that every programmer gets caught by it exactly two times. It is call the mutable defaults trap. Programmers are usually bit by the mutable defaults trap when coding class methods, but I’d like to begin with explaining it in functions, and then move on to talk about class methods.
Mutable defaults for function arguments
The gotcha occurs when you are coding default values for the arguments to a function or a method. Here is an example for a function named foobar:
def foobar(arg_string = "abc", arg_list = ): ...
Here’s what most beginning Python programmers believe will happen when foobar is called without any arguments:
A new string object containing “abc” will be created and bound to the “arg_string” variable name. A new, empty list object will be created and bound to the “arg_list” variable name. In short, if the arguments are omitted by the caller, the foobar will always get “abc” and  in its arguments.
This, however, is not what will happen. Here’s why.
The objects that provide the default values are not created at the time that foobar is called. They are created at the time that the statement that defines the function is executed. (See the discussion at Default arguments in Python: two easy blunders: “Expressions in default arguments are calculated when the function is defined, not when it’s called.”)
If foobar, for example, is contained in a module named foo_module, then the statement that defines foobar will probably be executed at the time when foo_module is imported.
When the def statement that creates foobar is executed:
- A new function object is created, bound to the name foobar, and stored in the namespace of foo_module.
- Within the foobar function object, for each argument with a default value, an object is created to hold the default object. In the case of foobar, a string object containing “abc” is created as the default for the arg_string argument, and an empty list object is created as the default for the arg_list argument.
After that, whenever foobar is called without arguments, arg_string will be bound to the default string object, and arg_list will be bound to the default list object. In such a case, arg_string will always be “abc”, but arg_list may or may not be an empty list. Here’s why.
There is a crucial difference between a string object and a list object. A string object is immutable, whereas a list object is mutable. That means that the default for arg_string can never be changed, but the default for arg_list can be changed.
Let’s see how the default for arg_list can be changed. Here is a program. It invokes foobar four times. Each time that foobar is invoked it displays the values of the arguments that it receives, then adds something to each of the arguments.
def foobar(arg_string="abc", arg_list = ): print arg_string, arg_list arg_string = arg_string + "xyz" arg_list.append("F") for i in range(4): foobar()
The output of this program is:
abc  abc ['F'] abc ['F', 'F'] abc ['F', 'F', 'F']
As you can see, the first time through, the argument have exactly the default that we expect. On the second and all subsequent passes, the arg_string value remains unchanged — just what we would expect from an immutable object. The line
arg_string = arg_string + "xyz"
creates a new object — the string “abcxyz” — and binds the name “arg_string” to that new object, but it doesn’t change the default object for the arg_string argument.
But the case is quite different with arg_list, whose value is a list — a mutable object. On each pass, we append a member to the list, and the list grows. On the fourth invocation of foobar — that is, after three earlier invocations — arg_list contains three members.
This behavior is not a wart in the Python language. It really is a feature, not a bug. There are times when you really do want to use mutable default arguments. One thing they can do (for example) is retain a list of results from previous invocations, something that might be very handy.
But for most programmers — especially beginning Pythonistas — this behavior is a gotcha. So for most cases we adopt the following rules.
- Never use a mutable object — that is: a list, a dictionary, or a class instance — as the default value of an argument.
- Ignore rule 1 only if you really, really, REALLY know what you’re doing.
So… we plan always to follow rule #1. Now, the question is how to do it… how to code foobar in order to get the behavior that we want.
Fortunately, the solution is straightforward. The mutable objects used as defaults are replaced by None, and then the arguments are tested for None.
def foobar(arg_string="abc", arg_list = None): if arg_list is None: arg_list =  ...
Another solution that you will sometimes see is this:
def foobar(arg_string="abc", arg_list=None): arg_list = arg_list or  ...
This solution, however, is not equivalent to the first, and should be avoided. See Learning Python p. 123 for a discussion of the differences. Thanks to Lloyd Kvam for pointing this out to me.
And of course, in some situations the best solution is simply not to supply a default for the argument.
Mutable defaults for method arguments
Now let’s look at how the mutable arguments gotcha presents itself when a class method is given a mutable default for one of its arguments. Here is a complete program.
# (1) define a class for company employees class Employee: def __init__ (self, arg_name, arg_dependents=): # an employee has two attributes: a name, and a list of his dependents self.name = arg_name self.dependents = arg_dependents def addDependent(self, arg_name): # an employee can add a dependent by getting married or having a baby self.dependents.append(arg_name) def show(self): print print "My name is.......: ", self.name print "My dependents are: ", str(self.dependents) #--------------------------------------------------- # main routine -- hire employees for the company #--------------------------------------------------- # (2) hire a married employee, with dependents joe = Employee("Joe Smith", ["Sarah Smith", "Suzy Smith"]) # (3) hire a couple of unmarried employess, without dependents mike = Employee("Michael Nesmith") barb = Employee("Barbara Bush") # (4) mike gets married and acquires a dependent mike.addDependent("Nancy Nesmith") # (5) now have our employees tell us about themselves joe.show() mike.show() barb.show()
Let’s look at what happens when this program is run.
- First, the code that defines the Employee class is run.
- Then we hire Joe. Joe has two dependents, so that fact is recorded at the time that the joe object is created.
- Next we hire Mike and Barb.
- Then Mike acquires a dependent.
- Finally, the last three statements of the program ask each employee to tell us about himself.
Here is the result.
My name is.......: Joe Smith My dependents are: ['Sarah Smith', 'Suzy Smith'] My name is.......: Michael Nesmith My dependents are: ['Nancy Nesmith'] My name is.......: Barbara Bush My dependents are: ['Nancy Nesmith']
Joe is just fine. But somehow, when Mike acquired Nancy as his dependent, Barb also acquired Nancy as a dependent. This of course is wrong. And we’re now in a position to understand what is causing the program to behave this way.
When the code that defines the Employee class is run, objects for the class definition, the method definitions, and the default values for each argument are created. The constructor has an argument arg_dependents whose default value is an empty list, so an empty list object is created and attached to the __init__ method as the default value for arg_dependents.
When we hire Joe, he already has a list of dependents, which is passed in to the Employee constructor — so the arg_dependents attribute does not use the default empty list object.
Next we hire Mike and Barb. Since they have no dependents, the default value for arg_dependents is used. Remember — this is the empty list object that was created when the code that defined the Employee class was run. So in both cases, the empty list is bound to the arg_dependents argument, and then — again in both cases — it is bound to the self.dependents attribute. The result is that after Mike and Barb are hired, the self.dependents attribute of both Mike and Barb point to the same object — the default empty list object.
When Michael gets married, and Nancy Nesmith is added to his self.dependents list, Barb also acquires Nancy as a dependent, because Barb’s self.dependents variable name is bound to the same list object as Mike’s self.dependents variable name.
So this is what happens when mutuable objects are used as defaults for arguments in class methods. If the defaults are used when the method is called, different class instances end up sharing references to the same object.
And that is why you should never, never, NEVER use a list or a dictionary as a default value for an argument to a class method. Unless, of course, you really, really, REALLY know what you’re doing.