If we assume a and b both contain 5 integers:
temp = [ x for x in zip(a, b) ]
What is the difference between these two pieces of code? Why does this work:
for q, p in temp:
pass
But not this?
q = [ x for x, _ in temp ]
p = [ x for _, x in temp ]
The second fails with ValueError: not enough values to unpack (expected 2, got 1).
Here's how it works.
Let's say
a = range(1,6)
b = range(3,8)
temp = [ x for x in zip(a, b) ]
Here, x will get a tuple. It will have each element from a and b sent to x. So the result of this will be:
[(1, 3), (2, 4), (3, 5), (4, 6), (5, 7)]
If you want each value from the for loop to be stored in different variables, you can give:
temp = [(p,q) for p,q in zip(a, b)]
Here, p will get values 1, 2, 3, 4, 5 and q will get values 3, 4, 5, 6, 7
The result of both the list comprehensions are the same.
Now let's look at the other two.
q = [ x for x, _ in temp ]
p = [ x for _, x in temp ]
Here, the code is trying to pick values from temp which has tuples. Each iteration, two values are retrieved. For the code
q = [ x for x, _ in temp ]
it is retaining the first value into x and discarding the second value. So list q will have [1, 2, 3, 4, 5].
Similarly, for p, it discards the first value while retaining the second value. So list p will have [3, 4, 5, 6, 7]
And lastly, when you look at
for q, p in temp:
pass
You are iterating through temp but you are not storing the values anywhere. So once all the values in temp are iterated thru, the values of q and p will be the last element of temp which will be (5,7). This will result in q getting 5 and p getting 7.
The first one is list comprehension and other one is simple for loop. List comprehension is an easy way to run some loop or some conditions and it rerurns a list.
So in your example, temp will be a list.
We also have dict comprehension which returns dictionary
