233 – Tracking generator expression progress

Assignment expressions (using the walrus operator :=) used inside generator expressions can be used to keep track of the progress of the generator itself:

_s = None
squares = ((_s := n) ** 2 for n in range(10))

The global variable _s is set to the value of n whenever the generator squares advances, as the following interaction shows:

print(_s)  # None
next(squares)
next(squares)
print(_s)  # 1
next(squares)
next(squares)
print(_s)  # 3

This interaction is not recommended for production code but it can be helpful when trying to debug a generator expression. An interesting example application is when using this technique with the built-ins all and any when they’re fed a generator expression:

numbers = [42, 0, 16, 73, 10]
all((odd := n) % 2 == 0 for n in numbers)
print(odd)  # 73

Since the built-ins all and any short-circuit, the variable odd will hold the first odd number from the list numbers. If the list numbers didn’t have any odd numbers, it would have the value of the last number in the list.