What does the “as” statement mean in python?

Question

I'm just starting to learn the Flask framework and was wondering what the 'as' statement does? It's used in conjunction with an 'with' statement.

Here's the example:

def init_db():
with closing (connect_db()) as db:
    with app.open_resource('schema.sql', mode='r') as f:
        db.cursor().executescript(f.read())
    db.commit

Answer 1

The as keyword is used to add clauses to a few different statements (eg, import ); there is no " as statement".

In the with statement, it means that the value of the with context gets assigned to that variable. The precise explanation is in the docs under The with statement , With Statement Context Managers , and Context Manager Types ; PEP 343 gives a more readable explanation (although it's also a little out of date).

---

In simple cases, where an object acts as its own context manager, as a file does, or decimal.localcontext , the object gets assigned to the variable. Here, f is the file returned by open('spam') :

with open('spam') as f:

In slightly more complex cases, a context manager provides some other object that gets assigned to the variable. In the case of closing(foo) , the object is the foo that it was given in the first place. So here, g ends up being the same thing as f , even though closing(f) is not the same thing:

f = open('spam')
with closing(f) as g:

Some context managers don't provide any object at all. In that case, as you'd expect, as f will assign f to None , and you usually won't have nay good reason to use it. So, the as clause is optional. For example, using a threading.Lock :

with my_lock:

---

If you're building context managers from scratch, the way you provide an object (whether self or otherwise) to bind to the as target is by returning it from the __enter__ method. Or, if you're building them with the @contextmanager decorator around a generator, you do it by yield ing the object.