How list of lists iteration is working in python?

Question

I have a list of list like this:-

lst = [[1, 2, 3, 4, 5, 6], [4, 5, 6], [7], [8, 9]]

If I run these I got output like these.I am not gettin

Answer 1

[j for i in list for j in i]

This is similar to

result = []
for i in list:
    for j in i:
        result.append(j)

In general the list comprehension like [p for a in b if c == d for e in f if etc] will be translated to like

reuslt = []
for a in b: if c == d: for e in f: if etc: result.append(p)

---

[j for j in i for i in list]

Normally this won't even run. Probably you have defined i to be [8, 9] previously.

>>> [j for j in i for i in list]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'i' is not defined

This is equivalent to

result = []
for j in i: 
    for i in list:
        result.append(j)

so if i is not defined at first, the loop will not work.

Answer 2

At the end of first LC i is assigned to [8,9]:

>>> lis = [[1, 2, 3, 4, 5, 6], [4, 5, 6], [7], [8, 9]]
>>> [j for i in lis for j in i]
[1, 2, 3, 4, 5, 6, 4, 5, 6, 7, 8, 9]
>>> i
[8, 9]

Now in the second LC you're iterating over this i:

>>> [j for j in i for i in lis]
[8, 8, 8, 8, 9, 9, 9, 9]

Both LC's are (roughly)equivalent to:

>>> lis = [[1, 2, 3, 4, 5, 6], [4, 5, 6], [7], [8, 9]]
>>> for i in lis:
...     for j in i:
...         print j,
...         
1 2 3 4 5 6 4 5 6 7 8 9
>>> i
[8, 9]
>>> for j in i:
...     for i in lis:
...         print j,
...         
8 8 8 8 9 9 9 9

---

This has been fixed in py3.x:

in particular the loop control variables are no longer leaked into the surrounding scope.

Demo(py 3.x):

>>> lis = [[1, 2, 3, 4, 5, 6], [4, 5, 6], [7], [8, 9]]
>>> [j for i in lis for j in i]
[1, 2, 3, 4, 5, 6, 4, 5, 6, 7, 8, 9]
>>> i
Traceback (most recent call last):
NameError: name 'i' is not defined

>>> j
Traceback (most recent call last):
NameError: name 'j' is not defined