Confused about Python's with statement

I saw some code in the Whoosh documentation:

with ix.searcher() as searcher:
    query = QueryParser("content", ix.schema).parse(u"ship")
    results = s开发者_Python百科earcher.search(query)

I read that the with statement executes __ enter__ and __ exit__ methods and they are really useful in the forms "with file_pointer:" or "with lock:". But no literature is ever enlightening. And the various examples shows inconsistency when translating between the "with" form and the conventional form (yes its subjective).

Please explain

• what is the with statement
• and the "as" statement here
• and best practices to translate between both forms
• what kinds of classes lend them to with blocks

Epilogue

The article on http://effbot.org/zone/python-with-statement.htm has the best explanation. It all became clear when I scrolled to the bottom of the page and saw a familiar file operation done in with. https://stackoverflow.com/users/190597/unutbu ,wish you had answered instead of commented.

Example straight from PEP-0343:

with EXPR as VAR:
   BLOCK

#translates to:

mgr = (EXPR)
exit = type(mgr).__exit__  # Not calling it yet
value = type(mgr).__enter__(mgr)
exc = True
try:
    try:
        VAR = value  # Only if "as VAR" is present
        BLOCK
    except:
        # The exceptional case is handled here
        exc = False
        if not exit(mgr, *sys.exc_info()):
            raise
        # The exception is swallowed if exit() returns true
finally:
    # The normal and non-local-goto cases are handled here
    if exc:
        exit(mgr, None, None, None)

Read http://www.python.org/dev/peps/pep-0343/ it explains what the with statement is and how it looks in try .. finally form.