`while` loops are not discussed anymore

[fm2003]

Thanks for Think Python! I've been using it for years to teach Python at the TU Delft (and I know several colleagues are using it too).

I'm now updating a course to use the 3rd edition instead of the 2nd edition, and I noticed that while loops are not discussed in the book anymore. Was there a particular reason for dropping this part? Would you consider including it again?

[AllenDowney]

I have to admit, it was not entirely a deliberate decision to leave them out. In the second edition, I thought the examples where I showed while loops were not so good, so I cut them with the intention of finding a better example. But a good opportunity never came up, and it slipped my mind.

You are not the first to notice, so I might add a section to the online edition. I'll look for a good place to include them -- do you have suggestions?

[fm2003]

I would say it belongs under "iteration" (where it also was in the 2nd edition). I personally didn't mind the "Newton's method" example, but I can also see how this does not fit the narrative of the new "Iteration and Search" chapter very well.

Thinking out loud, maybe you could do something like "Give me the first 5 lines of the file that contain an 'e'":

count = 0
with open('words.txt') as f:
    while (line := f.readline()) and count < 5:
        if has_e(line):
            print(line)
            count += 1

Or, without the walrus operator:

count = 0
with open('words.txt') as f:
    while count < 5:
        line = f.readline()
        if not line:
            break
        if has_e(line):
            print(line)
            count += 1

But that then requires break, which is also not in the book anymore... And if we're using break anyway, we might as well just use a for loop...

count = 0
with open('words.txt') as f:
    for line in f:
        if has_e(line):
            print(line)
            count += 1
        if count >= 5:
            break

So yeah, I don't have better suggestions right now, but maybe this gives some inspiration.

[AllenDowney]

Thanks!

[Paramon]

I would say it belongs under "iteration" (where it also was in the 2nd edition). I personally didn't mind the "Newton's method" example, but I can also see how this does not fit the narrative of the new "Iteration and Search" chapter very well.

Thinking out loud, maybe you could do something like "Give me the first 5 lines of the file that contain an 'e'":

count = 0
with open('words.txt') as f:
while (line := f.readline()) and count < 5:
if has_e(line):
print(line)
count += 1
Or, without the walrus operator:

count = 0
with open('words.txt') as f:
while count < 5:
line = f.readline()
if not line:
break
if has_e(line):
print(line)
count += 1
But that then requires break, which is also not in the book anymore... And if we're using break anyway, we might as well just use a for loop...

count = 0
with open('words.txt') as f:
for line in f:
if has_e(line):
print(line)
count += 1
if count >= 5:
break
So yeah, I don't have better suggestions right now, but maybe this gives some inspiration.

Alternatively, if you really wanted a version without an explicit counter, you could use a generator and itertools.islice like this:

from itertools import islice

with open('words.txt') as f:
    matches = (line for line in f if has_e(line))
    for line in islice(matches, 5):
        print(line)

Using while we have to handle StopIteration, otherwise the program will crash if the file contains fewer than 5 matching lines.

with open('words.txt') as f:
    matches = (line for line in f if has_e(line))
    count = 0
    while count < 5:
        try:
            print(next(matches))
            count += 1
        except StopIteration:
            break

Funny enough, I can’t even remember the last time I used a while loop. Thanks for reminding me about it today!

I am out. Whenever I try to optimize code, it usually turns into something nobody — including me — can read.

Here’s a one-liner version that prints the first 5 lines from a file that match has_e:

[print(line) for _, line in zip(range(5), (l for l in open('words.txt') if has_e(l)))]

Breakdown:

• open('words.txt') — opens the file (lazy evaluation via generator expression).

• if has_e(l) — filters lines.

• zip(range(5), ...) — limits to 5 matches.

• print(line) — prints each matching line.

• _ is a throwaway for the counter from range(5).

Note: This is more of a fun hack than a clean solution. It works, but for readability and safety, the earlier multi-line versions are way better.