iter function with sentinnel

Hello Pythonistas, did you know that Python iter function has a two arg variant ?

iter(func, sentinnel)

However the func - must be a zero argument callable.

for _ in iter(func, sentinnel):
    do_something()

In this case, loop will run only till func produces the sentinnel value.

Some use cases :

1. Run a game loop for specific duration
2. Read lines from file till particular match is found
3. Read data from socket in chunk sizes

In [14]: from functools import partial

In [15]: import random

In [16]: genrate_nums = partial(random.randint, 1, 10) # Generate random numbers between 1, 10

In [17]: genrate_nums() # a random number between 1 and 10 is returned.
Out[17]: 5

In [18]: genrate_nums()
Out[18]: 2

Lets look at these two runs. The varying number of iterations in two cases indicate that in the first run the sentinnel was returned from our generate_nums function when it was called 5th time and in the next run the setinnel value was returned when the generate_nums was called for 14th time.

In [23]: for i in iter(genrate_nums, 5):
    ...:     print(i)
    ...:

1
2
9
4

In [24]: for i in iter(genrate_nums, 5):
    ...:     print(i)
    ...:

4
10
9
3
2
8
1
9
3
7
8
3
2
1

Second Example :

Lets look at another example where we have a file called ‘hello.txt’ We wish to read lines in file will the line contains “hello” So effectively we wish to read only first two lines in this files.

Lets see how we can go about achieving the same.

In [14]: cat 'hello.txt'
sdfasd
sdfdfdf
hello
sdfasd
asdfasd

In [15]: with open('hello.txt') as file:
    ...:     for line in iter(file.readline, "hello\n"):
    ...:         print(line, end='')
    ...:
    ...:
sdfasd
sdfdfdf