What is the purpose of context manager in python

Question

I read http://eigenhombre.com/2013/04/20/introduction-to-context-managers/ .

In him:

Context managers are a way of allocating and releasing a resource exactly where you need it. The simplest example is file access:

with file("/tmp/foo", "w") as foo:
    print >> foo, "Hello!"

This is essentially equivalent to:

foo = file("/tmp/foo", "w")
try:
    print >> foo, "Hello!"
finally:
    foo.close()

The article provides further clarifications, but I'm still not sure that I understand their purpose in a simple way. Can someone clarify. And what is context?

I looked at Attempting to understand python using operators and context managers , but again I'm not sure what the purpose of the context manager is? Is this just an alternative syntax for "try .. finally .." or is it their other purpose.

Answer 1

- . ( ).

, :

f = open(path, "w")

. . , :

f.close()

:

f = open(path, "w")
data = 3/0  # Tried dividing by zero. Raised ZeroDivisionError
f.write(data)
f.close()

, , . (CPython , , )

A , , :

with open(path, "w") as f:
    data = 3/0  # Tried dividing by zero. Raised ZeroDivisionError
    f.write(data)
# In here the file is already closed automatically, no matter what happened.

---

with . : threading.Lock()

lock = threading.Lock()
with lock:  # Lock is acquired
   do stuff...
# Lock is automatically released.

, , try: ... finally: ..., , , __enter__ __exit__. .

---

__enter__() __exit__() .

__enter__() , , __exit__(), ( __exit__(), )

contextlib. .