The developer should not directly define a variable inside the “if statement” because if you are using that variable outside its scope, you will face a NameError exception.

Variables in if blocks follow the same scope as their enclosing function/module. 

For example, if you are initializing a variable inside the if block and printing its value outside of the scope of the if statement. Now, by chance, if the condition is not executed or False, the variable inside it will never be initialized.

From the above flowchart, you can see that if you try to use that variable outside the if block without ensuring it’s been initialized, you risk a NameError. Use else or initialize variables to avoid NameError.

if False:
    variable = 10
print(variable)  

# Output: NameError: name 'variable' is not defined. Did you mean: 'callable'?

In the above code, the condition will never be True, and hence the variable will never be initialized.

To initialize it, we need to satisfy the if condition like this:

if True:
    variable = 10
print(variable)

# Output: 10

Handling NameError exception

It is always a good practice not to use an if statement alone.

Use an if-else statement, so that when the if condition fails, you can use the else statement to initialize the variable, and after that, you can print it.

if False:
    variable = 10
else:
    variable = 20
print(variable)

# Output: 20

After else is executed, the variable is initialized, and now you can print its value outside of its scope.

Nested if Statements

If you are working with a nested if-else block, make sure that you define it with proper handlers.

if True:
    if False:
        value = "A"
    else:
        value = "B"
else:
    value = "C"

print(value)

# Output: "B"

Using Walrus Operator (:=)

If you are using Python 3.8 or above, you can assign variables directly in the condition of the if statement.

data = [1, 2, 3, 4, 5]

if (n := len(data)) > 4:
    print(f"Data has {n} elements")

# Output: Data has 5 elements

If the length of the data is less than 5, it won’t print anything. If the length is greater than 5, it will print its length.

You can access the variables assigned in the condition even if the block is not executed.

data = [1, 2, 3, 4, 5]

if (x := 0) > len(data):
    pass

print(x)
# Output: 0

Pitfalls in List Comprehensions

The main danger of the walrus operator in list comprehension is that it may leak variables into the outer scope.

data = [1, 2, 3, 4, 5]

squares = [x for num in data if (x := num) > 1]

print(x)
# Output: 5

Error Handling in If Blocks

To handle the potential errors, you can always use the try/except mechanism.

use_input = True

if use_input:
    try:
        value = int(input("Enter a number: "))
    except ValueError:
        value = 0
else:
    value = 42
print(value)  # Safe due to error handling

# Output:
# Enter a number: 10
# 10

That’s all!