How do I make a contextmanager with a loop inside?

问题

I want something like this:

from contextlib import contextmanager

@contextmanager
def loop(seq):
    for i in seq:
        try:
            do_setup(i)
            yield # with body executes here
            do_cleanup(i)
        except CustomError as e:
            print(e)

with loop([1,2,3]):
    do_something_else()
    do_whatever()

But contextmanager doesn't work because it expects the generator to yield exactly once.

The reason why I want this is because I basically want to make my own custom for loop. I have a modified IPython that is used to control test equipment. It's obviously a full Python REPL, but most of the time the user is just calling predefined functions (similar to Bash prompt), and the user is not expected to be a programmer or familiar with Python. There needs to be a way to loop over some arbitrary code with setup/cleanup and exception handling for each iteration, and it should be about as simple to type as the above with statement.

回答1:

I think a generator works better here:

def loop(seq):
    for i in seq:
        try:
            print('before')
            yield i  # with body executes here
            print('after')
        except CustomError as e:
            print(e)

for i in loop([1,2,3]):
    print(i)
    print('code')

will give:

before
1
code
after
before
2
code
after
before
3
code
after

Python enters and exits a with block only once so you can't have logic int the enter / exit steps that would be done repeatedly.

回答2:

A more complete answer, for if the exception might happen outside the generator:

from contextlib import contextmanager

class CustomError(RuntimeError):
    pass

@contextmanager
def handle_custom_error():
    try:
        yield
    except CustomError as e:
        print(f"handled: {e}")

def loop(seq):
    for i in seq:
        try:
            print('before')
            if i == 0:
                raise CustomError("inside generator")
            yield i # for body executes here
            print('after')
        except CustomError as e:
            print(f"handled: {e}")

@handle_custom_error()
def do_stuff(i):
    if i == 1:
        raise CustomError("inside do_stuff")
    print(f"i = {i}")

for i in loop(range(3)):
    do_stuff(i)

Output:

before
handled: inside generator
before
handled: inside do_stuff
after
before
i = 2
after

来源：https://stackoverflow.com/questions/29708445/how-do-i-make-a-contextmanager-with-a-loop-inside

标签

python

contextmanager