When you pass a list to a function, the function gets a reference to the list. If the function modifies a list parameter, the caller sees the change. For example,
delete_head removes the first element from a list:
def delete_head(t): del t
Here’s how it is used:
>>> letters = ['a', 'b', 'c'] >>> delete_head(letters) >>> print letters ['b', 'c']
t and the variable
letters are aliases for the same object. The stack diagram looks like Figure 10.12.1.
Since the list is shared by two frames, I drew it between them.
It is important to distinguish between operations that modify lists and operations that create new lists. For example, the
append method modifies a list, but the
+ operator creates a new list:
>>> t1 = [1, 2] >>> t2 = t1.append(3) >>> print t1 [1, 2, 3] >>> print t2 None >>> t3 = t1 +  >>> print t3 [1, 2, 3, 4]
This difference is important when you write functions that are supposed to modify lists. For example, this function does not delete the head of a list:
def bad_delete_head(t): t = t[1:] # WRONG!
The slice operator creates a new list and the assignment makes
t refer to it, but none of that has any effect on the list that was passed as an argument.
An alternative is to write a function that creates and returns a new list. For example,
tail returns all but the first element of a list:
def tail(t): return t[1:]
This function leaves the original list unmodified. Here’s how it is used:
>>> letters = ['a', 'b', 'c'] >>> rest = tail(letters) >>> print rest ['b', 'c']