Question

I am having some trouble getting expected results out of the zip command. Cases 1-3 make sense, but in cases 4 and 5 (which I assume are equivalent?) I expect the results to be [[‘a’],[‘b’],[‘c’],[‘d’]], but instead the entirety of the second list is appended to each sublist of the list of lists I initialize.

Case 1:

>>> for a in zip([1,2,3,4],['a','b','c','d']):
...     print a

(1, 'a')
(2, 'b')
(3, 'c')
(4, 'd')

Case 2:

>>> for (a,b) in zip([1,2,3,4],['a','b','c','d']):
...     print a,b
... 
1 a
2 b
3 c
4 d

Case 3:

>>> temp = [[]] * 4
>>> for (a,b) in zip([0,1,2,3],['a','b','c','d']):
...     temp[a] = b
... 
>>> temp
['a', 'b', 'c', 'd']

Case 4:

>>> temp = [[]] * 4
>>> for (a,b) in zip([0,1,2,3],['a','b','c','d']):
...     temp[a].append(b)
... 
>>> temp
[['a', 'b', 'c', 'd'], ['a', 'b', 'c', 'd'], ['a', 'b', 'c', 'd'], ['a', 'b', 'c', 'd']]

Case 5:

>>> temp = [[]] * 4
>>> for a,b in zip([0,1,2,3],['a','b','c','d']):
...     temp[a].append(b)
... 
>>> temp
[['a', 'b', 'c', 'd'], ['a', 'b', 'c', 'd'], ['a', 'b', 'c', 'd'], ['a', 'b', 'c', 'd']]

Answer 1

You’ve run into the way Python deals with variable names and references. It can be confusing if you have worked with C-style languages before, but makes a lot of sense if you don’t think that way!

Python has “things” and “names for things”. For example, x is a name, [] is a thing, and x=[] assigns the name x to the thing [].

The list multiplication syntax is a bit confusing with regards to names and things. Let’s expand it out to see how it works:

name = [x]*4

is the same as

_temp = x
name = [_temp, _temp, _temp, _temp]

With that in mind, the following code should make sense:

>>> x = []
>>> y = [x]*4
>>> y[0].append(1)
>>> y
[[1], [1], [1], [1]]

Why? Well, when you write y=[x]*4] you make a list of four names. But those are all names for the same thing! If you .append(1) to the object named by the first name, you change the object that all the names, well, name.

Now, just as before,

name = [[]]*4

is the same as

_temp = []
name = [_temp, _temp, _temp, _temp]

You haven’t explicitly created a new name. But you have done so implicitly! You only call [] once, so you only make one new list. You then make a list of four names, each of the new empty list you just made. So it should come as no surprise that all the names name the same thing!

---

How to fix this, you say? Well, you need to call [] once for each new list that you want. To do that, use a list comprehension:

[[] for _ in range(4)]

Note that _ is just another name, although it is conventionally used for things you don’t care about.