I am looking for beginner help with Python lists. I create two identical lists a and b, but in different ways. Then I try to alter one value in the lists, in the same way. Why am I getting different results for both lists?
See code:
a = [[0] * 3] * 4
b = [[0, 0, 0], [0, 0, 0], [0, 0, 0], [0, 0, 0]]
print(a, id(a))
print(b, id(b))
print(a == b)
a[0][0] = 4
b[0][0] = 4
print(a, id(a))
print(b, id(b))
print(a == b)
The result I want is done by:
b[0][0] = 4
but not by:
a[0][0] = 4
The two lists you create are not identical, and that's why you're seeing different results.
a = [[0]*3]*4 # four copies of the same list
b = [[0,0,0],[0,0,0],[0,0,0],[0,0,0]] # four different lists, all
# three containing zeros.
[id(x) for x in a]
[4953622320, 4953622320, 4953622320, 4953622320]
# note - all are the same instance.
[id(x) for x in b]
[4953603920, 4953698624, 4953602240, 4953592848]
# note - all are the different instances
b[0][0] = 4 # change just one list
[[4, 0, 0], [0, 0, 0], [0, 0, 0], [0, 0, 0]]
a[0][0] = 4 # change one list, but since a contains four references
# to this list it seems as if four lists were modified.
[[4, 0, 0], [4, 0, 0], [4, 0, 0], [4, 0, 0]]
One final note - it's a cool question, in my mind. Takes a second to understand what's going on :)
The answer by @Roy2012 is completely correct.
I would add to their answer and say that there is a way of creating a "list of distinct lists" that avoids writing out the entire thing (like you did for b). Here it is:
a = [[0 for _ in range(3)] for _ in range(4)]
This syntax [x for x in range(y)] is called a "list comprehension" and is the same as doing this:
a = []
for _ in range(4):
c = []
for _ in range(3):
c.append(0)
a.append(c)
Also note that there is an easier way of checking if 2 things are the same object in memory than using id. You can just use is. In the example in the OP, we can see a[2] is a[3] evaluates to True (so they have the same memory location) but b[2] is b[3] evaluates to False.
