[Tutor] Debugging While Loops for Control

[Peter Otten]

Alan Gauld wrote:

>>The "x for x in y:" syntax makes it harder to follow for learners,
> 
> Read about list comprehensions first.
> It helps if you studied sets in math at school. The format is
> somewhat like the math notation for defining a set. But FWIW it took me
> a long time to get used to that syntax too.

To grok list comprehensions it helps to write the equivalent for loops 
first. You can mechanically convert

items = []
for x in "abcd":
    if x < "d":
        if x != "b":
            items.append(x)

to

items = [x for x in "abcd" if x < "d" if x != "b"]

The for loops and ifs stay in place, the expression passed to append() in 
the innermost loop moves to the beginning. Another example:

items = []
for x in "abcd":
    if x < "d":
        for y in x + x.upper():
            if y != "A":
                items.append(y*2)

This becomes

items = [y*2 for x in "abcd" if x < "d" for y in x + x.upper() if y != "A"]

Real-world list comprehensions tend to be less complicated, but to 
understand them you just have to reverse the conversion:

items = [y*2 
for x in "abcd" 
    if x < "d" 
        for y in x + x.upper() 
            if y != "A"
                # append y*2
]