python 3 - x for x for loops - how do they work?

Question

Can someone please explain in detail the below type of for loop?

primes = [x for x in range(2, 50) if x not in noprimes]

or

le_list = [i for i in getfiles(path) if i != 'fred']

The bit i dont get is the i for i, or x for x. I do not understand what its actually saying.

variable for variable in a list if variable whatever.

I dont see why you need variable twice at the start. The for loop goes through each item in list anyway, how is it treated differently?

Answer 1

The reason for the apparently redundant extra mention of the variable x when writing x for x is that the first x does not need to be x . It just happens to be in the examples you give. Here are a few more examples which should clarify the difference between the first and second x in your question:

ones = [1 for x in range(10)]

This simply gives a list of 10 ones, the same as [1] * 10 .

squares = [x*x for x in range(10)]

This gives x squared for each x in the specified range.

In your example, the second x is the variable used by the for loop, and the first x is simply an expression, which happens in your case to be just x . The expression can be whatever you like, and does not need to be in terms of x .

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results = [expression for x in range(10)]

expression can include anything you like - a string, a calculation, a function - whatever you choose. If the expression happens to be just x then it looks unusual if you are not used to it, but it's the same as the following:

results = []
for x in range(10):
    results.append(expression)

Answer 2

It is a placeholder for a transformation of the element.

Suppose you wanted a list of primes multiplied by 2. Then you could replace the first x with x*2 .

primes = [x*2 for x in range(2, 50) if x not in noprimes]

You write the following to say that you do not want a transformation, and just original x .

primes = [x for x in range(2, 50) if x not in noprimes]

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Another perspective : Consider the x in the statement primes.append(x) . That is the same as the first x in the loop in your question. In fact, this is the exact same loop as the one above.

primes = []
for x in range(2,50):
   if x not in noprimes:
       primes.append(x)

Here is the other example, with x*2 .

primes = []
for x in range(2,50):
   if x not in noprimes:
       primes.append(x*2)

Answer 3

The good way to understand it is to read it a bit different. So if we take your piece of code:

primes = [x for x in range(2, 50) if x not in noprimes]

We can read is as:

Primes = All "x for" which "x in range(2,50)" but only "if x not in noprimes"

I hope this helps you understand the functionality better.

Answer 4

This syntax is called a "comprehension" (eg list comprehension, dictionary comprehension, etc.). It's a compact way of specifying the contents of the list/dictionary/other data structure programmatically. The Python docs explain it pretty well.