A context manager is an object that defines the runtime
context to be established when executing a with
statement. The context manager handles the entry into,
and the exit from, the desired runtime context for the execution
of the block of code. Context managers are normally invoked using
the with statement (described in section 7.5), but
can also be used by directly invoking their methods.
Typical uses of context managers include saving and
restoring various kinds of global state, locking and unlocking
resources, closing opened files, etc.
Enter the runtime context related to this object. The with
statement will bind this method's return value to the target(s)
specified in the as clause of the statement, if any.
__exit__(
self, exc_type, exc_value, traceback)
Exit the runtime context related to this object. The parameters
describe the exception that caused the context to be exited. If
the context was exited without an exception, all three arguments
will be None.
If an exception is supplied, and the method wishes to suppress the
exception (i.e., prevent it from being propagated), it should return a
true value. Otherwise, the exception will be processed normally upon
exit from this method.
Note that __exit__ methods should not reraise the passed-in
exception; this is the caller's responsibility.
