在Lambda中使用上下文管理器，如何？

Question

How can I use a context manager in a lambda? Hacks accepted. Defer opinions about this being a bad usage of lambdas.

I know I can do this:

def f():
    with context():
        return "Foo"

But I would like to do something like this:

lambda: with context(): "Foo"

Answer 1

You can't replace the work with does with an expression, no. There are no hacks to get you there either, because there is no way to handle exceptions and finalisation within an expression.

That's because you can only use one expression in a lambda. with is a statement , not an expression. You'd have to replace that with exception handling ( try..except..finally ) and calls to the __enter__ and __exit__ methods (storing the __exit__ method first ). However, exception handling can only be done with statements, because an exception ends the current expression immediately. See Python Try Catch Block inside lambda .

Your only option is to stick to using a proper function instead.

Answer 2

One possible workaround for getting lambdas working with a context manager is to make the context manager a ContextDecorator , then both with statements and lambda expressions will work because a lambda can use the decorator pattern instead.

Example

from contextlib import ContextDecorator


def f(x):
     """Just prints the input, but this could be any arbitrary function."""
     print(x)


class mycontext(ContextDecorator):
    def __enter__(self):
        f('Starting')
        return self

    def __exit__(self, *exc):
        f('Finishing')
        return False

with mycontext():
    f('The bit in the middle')

mycontext()(lambda: f('The bit in the middle'))()