Understanding List Comprehensions In Python

When reading the official tutorial, I encountered this example: >>> vec = [[1,2,3], [4,5,6], [7,8,9]] >>> [num for elem in vec for num in elem] [1, 2, 3, 4, 5, 6,

Solution 1:

The loops in list comprehension are read from left to right. If your list comprehension would be written as ordinary loop it would look something like this:

>>>vec = [[1,2,3], [4,5,6], [7,8,9]]>>>l = []>>>for elem in vec:...for num in elem:...        l.append(num)...>>>l
[1, 2, 3, 4, 5, 6, 7, 8, 9]

In Python 2 the variables within the list comprehension share the outer scope so num is available to be used later:

>>>vec = [[1,2,3], [4,5,6], [7,8,9]]>>>[num for elem in vec for num in elem]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>>num
9

Note that on Python 3 behavior is different:

>>> vec = [[1,2,3], [4,5,6], [7,8,9]]
>>> [num for elem in vec for num in elem]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> num
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'num' is not defined

Solution 2:

No worries :).

When you execute the list comprehension the value of num is 9 so the next time you iterate through the vec. You will get a list of 9.

See this.

In [1]: vec = [[1,2,3], [4,5,6], [7,8,9]]
In [2]: [num for elem in vec for num in elem]
Out[2]: [1, 2, 3, 4, 5, 6, 7, 8, 9]
In [3]: num
Out[3]: 9
In [4]: [num for elem in vec]
Out[4]: [9, 9, 9]

Solution 3:

list1 = [num forelemin vec fornumin elem]

#is equivalent to:

list1 = []
forelemin vec:
    fornumin elem:
       list1.append(num)

Solution 4:

Let me try to make the answer more clears. And the order obviously will be from left to right and most right value will be stored in the variable i.e num and elem.

Initial Data:

vec = [[1,2,3], [4,5,6], [7,8,9]]
num  # Undefined
elem # Undefined

Step-1: After execution of line [num for elem in vec for num in elem]

# Now the value of undefined variable will benum = 9# will keep the last value of the loop as per python 2.7elem = [7, 8, 9]   # Same applies here (as the loop for elem in vec will get executed first followed by for num in elem)

Step-2: After execution of line [num for elem in vec]

# Output: [9, 9, 9]# Since num value is 9 and its get repeated 3 times because of vec has 3 elements (three list object in a list, so for loop will run 3 times)# Now the variable value would benum = 9# no changeelem = [7, 8, 9] # The last tuple of variable vec

Step-3: After the execution of [num for elem in (vec for num in elem)]

1.In the first/right loop i.e (vec for num in elem)
    Here the result will be a generator that will have run thrice since length of elem is3.2.Finalfor loop will iterate over RESULT1 (the resultoffor loop #1having length 3) and since the num valueis9. the result will be [9, 9, 9] # num value repeated thrice.