Python: Short circuit evaluation

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Division (or remainder) by zero causes a run-time error and a program crash (for a C program).

What happens when the following program is run? There are three choices.

The text "YES" is output.
The text "NO" is output.
The program crashes.

Here is the Python code [#1]

x1 = 156
y1 = 16
d1 = 0
if (x1 == y1) and ((x1 / d1) == 10):
	print("YES")
else:
	print("NO")

Here is the output of the Python code.

NO

What happened to the program crash?

In most programming languages, when one uses the conjunction operator or the disjunction operator a short-circuit evaluation takes place.

Short-circuit operators in Python:

conjunction: "and" (logical or)
disjunction: "or" (logical and)

In a conjunction, if the first argument is false, then the entire conjunction is false, so there is no need to evaluate the second argument. But that argument might crash the program or have side-effects.

In a disjunction, if the first argument is true, then the entire disjunction is true, so there is no need to evaluate the second argument. But that argument might crash the program or have side-effects.

The above program is equivalent to the following program - which makes the short-circuit behavior more apparent. Here is the Python code [#2]

x1 = 156
y1 = 16
d1 = 0
if x1 == y1:
	if (x1 / d1) == 10:
		print("YES")
	else:
		print("NO")
else:
	print("NO")

Here is the output of the Python code.

NO

Here is the type of program but using disjunction instead of conjunction.

Here is the Python code [#3]

x1 = 156
y1 = 156
d1 = 0
if (x1 == y1) or ((x1 / d1) == 10):
	print("YES")
else:
	print("NO")

Here is the output of the Python code.

YES

Here is the above program expanded to show the short-circuit evaluation.

Here is the Python code [#4]

x1 = 156
y1 = 156
d1 = 0
if x1 == y1:
	print("YES")
else:
	if (x1 / d1) == 10:
		print("YES")
	else:
		print("NO")

Here is the output of the Python code.

YES

Whenever one writes "and" (in Python) or "or" (in Python) one must be aware (in the mind) of how that expression is evaluated in terms of the expanded program code.

Since it can be hard to think about this accurately (and quickly) it is often best to avoid the use of "and" and/or "or" in Python unless necessary and/or you know what you are doing.