Ethereum: Boolean If statement inside a loop – a confusing bug
As a developer working with Ethereum-based smart contracts, you’re not alone in facing a frustrating bug that has left many confused. The issue in question is seemingly harmless: a “NoneType” error when trying to use a boolean value inside a loop.
The problem:
Using a boolean variable inside a loop can cause the entire expression to evaluate to False (or None in Python), regardless of the values assigned to the variables. This leads to unexpected behavior and incorrect results.
Sample Code:
Let’s look at a sample code snippet that illustrates this issue:
test_loop() definition:
bool_var = True
for _ in range(10):
if bool_var:
print("Loop iteration:", _)
In this example, we define a boolean variable bool_var and use it inside a loop. However, the expression if bool_var: evaluates to False (or None) if bool_var is set to True, causing the loop to skip each iteration.
The “NoneType” error:
As you might imagine, this behavior does not exactly match the expected result of the loop. The loop should continue until a condition is met or a maximum number of iterations is reached. However, if you use a boolean expression inside a loop, all iterations will eventually evaluate to False (or None), resulting in a false None error.
Solutions:
Fortunately, there are several solutions to this problem:
- Use the “any()” function: Instead of checking the entire condition with “if bool_var:”, use “any(bool_var for _ in range(10))”, which evaluates to “True” only if at least one iteration is successful.
test_loop() definition:
bool_var = True
for _ in range(10):
if(bool_var):
print("Loop iteration:", _)
- Use the
all()function: Similar to the previous solution, you can useall(bool_var for _ in range(10))which will evaluate to True only if all iterations are successful.
test_loop() definition:
bool_var = True
for _ in range(10):
if bool_var:
print("Loop iteration:", _)
- Avoid using booleans inside loops
: When possible, it is usually better to use other control structures, such as
iforelifstatements, to make your code more readable and maintainable.
test_loop() definition:
for _ in range(10):
if not bool_var:
Notice the change here!print("Loop iteration:", _)
By applying these solutions, you should be able to avoid the frustrating “NoneType” error when using boolean values inside loops. If your code continues to give you problems, provide more context or details for further assistance.