Error with context manager definition

Question

I have the following Python script:

from contextlib import contextmanager

@contextmanager
def my_content_manager():
    self.entrance = True

    try:
        yield
    except Exception as ex:
        self.entrance = False

with my_content_manager() as cm:
    print (cm.entrance)

print (cm.entrance)

When I tried to run the script, I got the following:

Traceback (most recent call last):
  File "test.py", line 12, in <module>
    with my_content_manager() as cm:
  File "C:\Users\abc\AppData\Local\Programs\Python\Python36\lib\contextlib.py", line 82, in __enter__
    return next(self.gen)
  File "test.py", line 5, in my_content_manager
    self.entrance = True
NameError: name 'self' is not defined

Why is that? How can I solve this error?

Thanks.

Well, what is `self` supposed to be referring to here? Where should it come from? Why do you think you need it at all? — Daniel Roseman, Nov 10 '16 at 23:12
If I remove "self", I get AttributeError: 'NoneType' object has no attribute 'entrance'. I'm basically trying to print True and False in the function. Any thoughts? Thanks — Simplicity, Nov 10 '16 at 23:15
The `AttributeError` makes sense because after you remove "self" your yield statement implicitly returns None (`cm` is that value and it does not have an attribute named "entrance"). — rkersh, Nov 11 '16 at 00:34

score 3 · Accepted Answer · edited May 23 '17 at 12:26

In general, using a class is probably a better way to go (as the other answer suggests). However, I remembered that function objects can have attributes. I came across this related question, which lead to the following trick (which does seem like an abuse of the feature):

from contextlib import contextmanager

@contextmanager
def my_content_manager():
    my_content_manager.entrance = True
    try:
        yield my_content_manager.entrance
    finally:
        my_content_manager.entrance = False
my_content_manager.entrance = False

with my_content_manager() as cm:
    print(my_content_manager.entrance)
    # Or, print(cm)

print(my_content_manager.entrance)

There are probably pitfalls to this technique. I'm simply posting it as a curiosity.

Pitfalls: not threadsafe, and not reentrant. Only one use of this context manager can be happening at a time. — user2357112, Nov 11 '16 at 00:09

Alex Hall · Answer 2 · 2016-11-10T23:26:35.707

2

The error NameError: name 'self' is not defined is pretty self-explanatory, no pun intended. self is just a name used by convention in methods of classes. You haven't defined self anywhere so Python doesn't know what to do. Moreover you've defined a function, not a method in a class, so this isn't close to working. Use a class to define your context manager like so:

class MyContextManager(object):
    def __init__(self):
        self.entrance = True

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        if exc_val:
            self.entrance = False
        # signal that the exception was handled and the program should continue
        return True


with MyContextManager() as cm:
    print (cm.entrance)
    raise Exception()

print (cm.entrance)

EDIT: if, as requested in a comment, you only want to print a value and not store it somewhere:

@contextmanager
def my_content_manager():
    try:
        yield
    except Exception as ex:
        print(False)
    else:
        print(True)

edited Nov 10 '16 at 23:26

answered Nov 10 '16 at 23:13

Alex Hall

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Thanks for your answer. I know how to use a class, but trying to use @contextlib instead. – Simplicity Nov 10 '16 at 23:17
@Simplicity well I've shown you how now. I don't think `contextlib` is appropriate for your use case. – Alex Hall Nov 10 '16 at 23:19
contextlib is another way to create context managers. I defined that through a class, and now trying to define an equivalent using @contextlib. My question is NOT about using a class to define a context manager. Does that make sense? Thanks – Simplicity Nov 10 '16 at 23:20
3

@Simplicity I am very familiar with `contextlib.contextmanager` and I'm telling you I don't think you can use it in this case. You want to store some data, you need a class. – Alex Hall Nov 10 '16 at 23:21
Say I just need to print True and False. That is, just print statements, cannot that be done using contextlib.contextmanager? – Simplicity Nov 10 '16 at 23:24

Error with context manager definition

2 Answers2