How to handle swapping variables in a pythonic way?

Question

I often have the case where I use two variables, one of them being the "current" value of something, another one a "newly retrieved" one.

After checking for equality (and a relevant action taken), they are swapped. This is then repeated in a loop.

import time
import random

def get_new():
    # the new value is retrieved here, form a service or whatever
    vals = [x for x in range(3)]
    return random.choice(vals)

current = None
while True:
    # get a new value
    new = get_new()
    if new != current:
        print('a change!')
    else:
        print('no change :(')
    current = new
    time.sleep(1)

This solution works but I feel that it is a naïve approach and I think I remember (for "write pythonic code" series of talks) that there are better ways.

What is the pythonic way to handle such mechanism?

Note that they aren't actually swapped; new always gets a fresh value, and current just gets the value from new, never vice versa. — chepner
– chepner, Commented Apr 19, 2018 at 15:52
The Pythonic idiom for swapping is a,b = b,a. But as @chepner pointed out, your code doesn't actually perform any swapping. — John Gordon
– John Gordon, Commented Apr 19, 2018 at 15:56

chepner · Accepted Answer · 2018-04-19 16:19:34Z

2

Really, all you have is a simple iteration over a sequence, and you want to detect changes from one item to the next. First, define an iterator that provides values from get_new:

# Each element is a return value of get_new(), until it returns None.
# You can choose a different sentinel value as necessary.
sequence = iter(get_new, None)

Then, get two copies of the iterator, one to use as a source for current values, the other for new values.

i1, i2 = itertools.tee(sequence)

Throw out the first value from one of the iterators:

next(i2)

Finally, iterate over the two zipped together. Putting it all together:

current_source, new_source = tee(iter(get_new, None))
next(new_source)
for current, new in zip(current_source, new_source):
    if new != current:
        ...
    else:
        ...
    time.sleep(1)

Using itertoolz.cons:

current_source, new_source = tee(iter(get_new, None))
for current, new in zip(cons(None, current_source), new_source)):
    ...

edited Apr 19, 2018 at 16:19

answered Apr 19, 2018 at 15:57

chepner

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2 Comments

chepner Over a year ago

I think there's room for improvement here, specifically replacing the explicit call to next with a clean way to prepend None to current_source instead: current_source = chain([None], current_source) (using the prepend recipe from the itertools documentation. YMMV.

chepner Over a year ago

The 3rd-party itertoolz module does provide cons as an implementation of the prepend recipe: current_source = cons(None, current_source).

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