CODEWITHSUNDEEP
pythongeneratorlist-comprehension
Loopz
Loopz

Reputation: 199

The order of nested list comprehension and nested generator expression in python

I'm new to Python and is confused by a piece of code in Python's official documentation.

unique_words = set(word  for line in page  for word in line.split())

To me, it looks equivalent to:

unique_words=set()
for word in line.split():
    for line in page:
        unique_words.add(word)

How can line be used in the first loop before it's defined in the nested loop? However, it actually works. I think it suggests the order of nested list comprehension and generator expression is from left to right, which contradicts with my previous understanding.

Can anyone clarify the correct order for me?

Upvotes: 6

Views: 8053

Answers (6)

Ahmad Shapiro
Ahmad Shapiro

Reputation: 1

for outer_val in outer_loop :
    for inner_val in inner_loop:
        do_something()

Translates to [do_something() for inner_val in inner_loop for outer_val in outer_loop ]

[ op <inner_loop> <outer_loop>]

Upvotes: 0

Vishnu Upadhyay
Vishnu Upadhyay

Reputation: 5061

word for line in page for word in line.split()

this part works like this:-

for line in page:
    for word in line.split():
        print word

() this makes it `generator function hence overall statement work lie this:-

def solve():
    for line in page:
        for word in line.split():
            yield word

and set() is used to avoid duplicacy or repetition of same word as the code is meant to get 'unique words'.

Upvotes: 7

John Y
John Y

Reputation: 14519

From the tutorial in the official documentation:

A list comprehension consists of brackets containing an expression followed by a for clause, then zero or more for or if clauses. The result will be a new list resulting from evaluating the expression in the context of the for and if clauses which follow it. For example, this listcomp combines the elements of two lists if they are not equal: 
>>> [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]
[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
and it’s equivalent to: 
>>> combs = []
>>> for x in [1,2,3]:
...     for y in [3,1,4]:
...         if x != y:
...             combs.append((x, y))
...
>>> combs
[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
Note how the order of the for and if statements is the same in both these snippets.

See the last sentence quoted above.

Also note that the construct you're describing is not (officially) called a "nested list comprehension". A nested list comprehension entails a list comprehension which is within another list comprehension, such as (again from the tutorial):

[[row[i] for row in matrix] for i in range(4)]

The thing you're asking about is simply a list comprehension with multiple for clauses.

Upvotes: 2

user3378649
user3378649

Reputation: 5354

In addition to the right answers that stressed the point of the order, I would add the fact that we use set to delete duplicates from line to make "unique words". check this and this thread

unique_words = set(word for line in page for word in line.split())
print unique_words

l = {}
for line in page:
    for word in line.split():
        l.add(word)
print l

Upvotes: -2

Yair Daon
Yair Daon

Reputation: 1123

You have the nested loops mixed. What the code does is:

unique_words={}
for line in page:
    for word in line.split():
        unique_words.add(word)

Upvotes: 0

Vincent Beltman
Vincent Beltman

Reputation: 2104

You got the loops wrong. Use this:

unique_words = set(word for line in page for word in line.split())
print unique_words

l = []
for line in page:
    for word in line.split():
        l.append(word)
print set(l)

output:

C:\...>python test.py
set(['sdaf', 'sadfa', 'sfsf', 'fsdf', 'fa', 'sdf', 'asd', 'asdf'])
set(['sdaf', 'sadfa', 'sfsf', 'fsdf', 'fa', 'sdf', 'asd', 'asdf'])

Upvotes: 2

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