Скачать или смотреть Resolving the Issue of lambda Expressions Not Finding Closed-Over Functions in Python 3.7

Discover how to resolve the `NameError` in Python 3.7 related to lambda expressions by understanding local and global scope nuances.
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Resolving the Issue of lambda Expressions Not Finding Closed-Over Functions in Python 3.7

When working with Python, you might encounter peculiarities that can lead to frustrating errors, especially when using lambda expressions in conjunction with function definitions. A common issue arises when trying to reference a function defined locally within a lambda, which can lead to a NameError when executing your code. Let's delve into this problem and see how you can resolve it.

The Problem: Missing Closure with Lambda Expressions

Consider the following code snippet:

[[See Video to Reveal this Text or Code Snippet]]

This code is intended to define a simple function bump and then use a lambda expression to call this function. However, when you execute this code, instead of printing 42, you get an error that suggests bump is not defined:

[[See Video to Reveal this Text or Code Snippet]]

Understanding the Cause of the Error

The root of the problem lies in how Python resolves the scope of variables. When a lambda function is created, it relies on the global scope to find free variables (like bump in our case). However, bump is defined in the local scope of test_compile_with_lambda. When exec() runs the compiled code, it looks for bump globally, which is why it fails to find it.

The Solution: Passing Local Scope to exec()

To solve this problem, you need to explicitly tell exec() to use the local scope. By passing locals() as the second argument to exec(), the lambda will have access to the local variables, including bump. Here's how you can modify your existing code:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Modified Code

Using locals(): The change in the exec function call allows the lambda expression to access bump, as it now searches both the local and global scope for bump.

Expected Output: With this change, when you run test_compile_with_lambda(), you will receive the expected output of 42.

Conclusion

In Python, understanding the difference between local and global scopes is crucial, especially when dealing with function definitions and lambda expressions. Always ensure that the encapsulated functions or variables are accessible within the scope where they are being called. By leveraging the locals() function, you can make sure that your lambda expressions have the context they need, solving the NameError and allowing your Python code to execute smoothly.

By keeping these principles in mind, you'll be better equipped to write clear and functional Python code without running into scoping issues. Happy coding!