CODEWITHSUNDEEP
pythonpython-3.xcontextmanager
user3814582
user3814582

Reputation: 153

Returning "self"? what does it really do and when do we need to return self

I encountered a code which looks similar to this:

from contextlib import contextmanager, ContextDecorator

class makepara(ContextDecorator):
     def __enter__(self):
        print ("<p>")
        return self

     def __exit__(self, *args):
         print ("</p>")
         return False

@makepara()
def emit_data():
    print (" here is HTML code")

    emit_data()

I found related answer this but when i change the above code to 

from contextlib import contextmanager, ContextDecorator

class makepara(ContextDecorator):
    def __enter__(self):
        print ("<p>")

    def __exit__(self, *args):
        print ("</p>")

@makepara()
def emit_data():
    print (" here is HTML code")

emit_data()

there is no change in the output, that makes me wonder what does return self actually does and how to be used?

Upvotes: 1

Views: 438

Answers (2)

Sraw
Sraw

Reputation: 20224

return self is not only useful in with statement, but also useful in many other situations.

For example, when you open a file using:

with open("file") as f:
    ....

Function open actually return an object which implements __enter__, and in its __enter__, it use return self to let you bind this instance to variable f, so that you can do f.read or something else after.

In other situations, for another example, if you want to chained call(Maybe data = a.connect().get("key").to_dict()). You need to add return self to connect and get.

But after all, return self is nothing more than returning a normal variable.

Upvotes: 2

wim
wim

Reputation: 363053

You choose to return self (or some other object, but usually the context manager instance itself) so that a name can be bound with this syntax:

with makepara() as var:
    ...

The object returned by __enter__ will be bound to the name var within the context (and will, in fact, remain bound to var after exiting context).

If you wouldn't need any value bound after entering context, it is possible to omit an explicit return (the implicit return of None would be used in this case regardless) but there is no harm and no disadvantage in returning self anyway.

Upvotes: 4

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