bind loop variable value to lambda

Question

Python is not fixing the value of the loop variable when passing it to a lambda.

xs = ["a", "b", "c"]
print_statements: List[Callable[[], None]] = []
for x in xs:
    print_statements.append(lambda: print(f"The value: {x}"))

for print_statement in print_statements:
    print_statement()

// Output:
// The value: c
// The value: c
// The value: c

Consider the same example in Java:

import java.util.ArrayList;
import java.util.List;

public class Main {
    public static void main(String[] args)
    {
        List<String> xs = List.of("a", "b", "c");
        List<Runnable> printStatements = new ArrayList<>();
        for (String x : xs)
        {
            printStatements.add(() -> System.out.println("The value: " + x));
        }

        for (Runnable printStatement : printStatements)
        {
            printStatement.run();
        }
    }
}

// Output:
// The value: a
// The value: b
// The value: c

I have more experience in Java. Thus, the output of the Java program is what I have expected. This is a real gotcha for me in Python and I would like to understand what is happening.

What is going on in the Python example?
- It seems like the reference to the loop variable is passed
- Is there maybe lazy evaluation going on, which causes this behavior?
Why has Python been implemented this way? What is the reasoning behind it by the Python devs?
Could you point me to Python documentation that covers the relevant scoping rules?
How can I change above Python Code to get the same output as in the Java example

mihca · Answer 1 · 2021-08-09T19:15:42.550

How can I change above Python Code to get the same output as in the Java example

One can bind the value of the outer scope variable to the scope of the lambda:

xs = ["a", "b", "c"]
print_statements: List[Callable[[], None]] = []
for x in xs:
    print_statements.append(lambda local_x=x: print(f"The value: {local_x}"))

# Output:
# The value: a
# The value: b
# The value: c

Still I wonder why Python has been designed this way.

I just noticed that Java will not let you do the following:

for (int i = 0; i < xs.size(); i++)
{
    printStatements.add(() -> System.out.println("The value: " + xs.get(i)));
}

The compiler complaints: Variable used in lambda expression should be final or effectively final

A fix for this is then

for (int i = 0; i < xs.size(); i++)
{
    int finalI = i;
    printStatements.add(() -> System.out.println("The value: " + xs.get(finalI)));
}

I think this corresponds to the Python code. Python is less strict and allows (in Java terms) to use a variable that is not "final or effectively final" to be used in a lambda.

bind loop variable value to lambda

1 Answers1