python expression

Question

I am new in python, and while reading a BeautifulSoup tutorial, I didn't understand this expression "[x for x in titles if x.findChildren()][:-1]" that i didn't understand? can you explain it

titles = [x for x in titles if x.findChildren()][:-1]

Answer 1

To start with [:-1], this extracts a list that contains all elements except the last element.

>>> a=[1,2,3,4,5]
>>> a[:-1]
[1, 2, 3, 4]

The comes the first portion, that supplies the list to [:-1] (slicing in python)

[x for x in titles if x.findChildren()]

This generates a list that contains all elements (x) in the list "titles", that satisfies the condition (returns True for x.findChildren())

Answer 2

It's called a for comprehension expression. It is simply constructing a list of all titles in the x list which return true when the findChildren function is called upon them. The final statement substracts the last one from the list.

Answer 3

It's a list comprehension.

It's pretty much equivalent to:

def f():
    items = []
    for x in titles:
        if x.findChildren():
            items.append(x)
    return items[:-1]
titles = f()

One of my favorite features in Python :)

Answer 4

The expression f(X) for X in Y if EXP is a list comprehension It will give you either a generator (if it's inside ()) or a list (if it's inside []) containing the result of evaluating f(X) for each element of Y, by only if EXP is true for that X.

In your case it will return a list containing every element from titles if the element has some children.

The ending [:-1] means, everything from the list apart from the last element.