Question

In this question, I defined a context manager that contains a context manager. What is the easiest correct way to accomplish this nesting? I ended up calling self.temporary_file.__enter__() in self.__enter__(). However, in self.__exit__, I am pretty sure I have to call self.temporary_file.__exit__(type_, value, traceback) in a finally block in case an exception is raised. Should I be setting the type_, value, and traceback parameters if something goes wrong in self.__exit__? I checked contextlib, but couldn’t find any utilities to help with this.

Original code from question:

import itertools as it
import tempfile

class WriteOnChangeFile:
    def __init__(self, filename):
        self.filename = filename

    def __enter__(self):
        self.temporary_file = tempfile.TemporaryFile('r+')
        self.f = self.temporary_file.__enter__()
        return self.f

    def __exit__(self, type_, value, traceback):
        try:
            try:
                with open(self.filename, 'r') as real_f:
                    self.f.seek(0)
                    overwrite = any(
                        l != real_l
                        for l, real_l in it.zip_longest(self.f, real_f))
            except IOError:
                overwrite = True
            if overwrite:
                with open(self.filename, 'w') as real_f:
                    self.f.seek(0)
                    for l in self.f:
                        real_f.write(l)
        finally:
            self.temporary_file.__exit__(type_, value, traceback)

Answer 1

The easy way to create context managers is with contextlib.contextmanager. Something like this:

@contextlib.contextmanager
def write_on_change_file(filename):
    with tempfile.TemporaryFile('r+') as temporary_file:
        yield temporary_file
        try:
             ... some saving logic that you had in __exit__ ...

Then use with write_on_change_file(...) as f:.
The body of the with statement will be executed “instead of” the yield. Wrap the yield itself in a try block if you want to catch any exceptions that happen in the body.

The temporary file will always be properly closed (when its with block ends).