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Python FTW: Under the Hood

Evaluation Time Discrepancy

Discover the differences between compile-time and runtime evaluations in Python generators. Learn how generator expressions handle variable references and updates, enabling you to predict outputs and avoid common pitfalls in Python programming.

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Do the compile-time and runtime evaluations differ? Let’s find out.

1.

Let's see an example for a typical generator expression:
C++
array = [1, 8, 15]
gen = (x for x in array if array.count(x) > 0)
array = [2, 8, 22]
print(list(gen)) # Where did the other values go?

2.

Let's modify the code a bit.
C++
array_1 = [1,2,3,4]
gen_1 = (x for x in array_1)
array_1 = [1,2,3,4,5]
print(list(gen_1))
array_2 = [1,2,3,4]
gen_2 = (x for x in array_2)
array_2[:] = [1,2,3,4,5]
print(list(gen_2))

3.

Can you predict the output of the code below?

C++
array_3 = [1, 2, 3]
array_4 = [10, 20, 30]
gen = (i + j for i in array_3 for j in array_4)
array_3 = [4, 5, 6]
array_4 = [400, 500, 600]
print(list(gen))

Explanation

  • In a generator expression, the in clause is evaluated at declaration time, but the conditional clause is
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