Is it possible to declare more than one variable using a with statement in Python?
Something like:
from __future__ import with_statement
with open("out.txt","wt"), open("in.txt") as file_out, file_in:
for line in file_in:
file_out.write(line)
... or is cleaning up two resources at the same time the problem?
It is possible in Python 3 since v3.1 and Python 2.7. The new with syntax supports multiple context managers:
with A() as a, B() as b, C() as c:
doSomething(a,b,c)
Unlike the contextlib.nested, this guarantees that a and b will have their __exit__()'s called even if C() or it's __enter__() method raises an exception.
You can also use earlier variables in later definitions (h/t Ahmad below):
with A() as a, B(a) as b, C(a, b) as c:
doSomething(a, c)
As of Python 3.10, you can use parentheses:
with (
A() as a,
B(a) as b,
C(a, b) as c,
):
doSomething(a, c)
with open('./file') as arg.x = file:? –
Eberhardt as is optional. –
Exudation as is required if you need the object a or b, but the whole as a or as b is not required –
Missie Note that if you split the variables into lines, prior to Python 3.10 you must use backslashes to wrap the newlines.
with A() as a, \
B() as b, \
C() as c:
doSomething(a,b,c)
Parentheses don't work, since Python creates a tuple instead.
with (A(),
B(),
C()):
doSomething(a,b,c)
Since tuples lack a __enter__ attribute, you get an error (undescriptive and does not identify class type):
AttributeError: __enter__
If you try to use as within parentheses, Python catches the mistake at parse time:
with (A() as a,
B() as b,
C() as c):
doSomething(a,b,c)
SyntaxError: invalid syntax
This issue is tracked in https://bugs.python.org/issue12782.
Python announced in PEP 617 that they would replace the original parser with a new one. Because Python's original parser is LL(1), it cannot distinguish between "multiple context managers" with (A(), B()): and "tuple of values" with (A(), B())[0]:.
The new parser can properly parse multiple context managers surrounded by parentheses. The new parser has been enabled in 3.9. It was reported that this syntax will still be rejected until the old parser is removed in Python 3.10, and this syntax change was reported in the 3.10 release notes. But in my testing, it works in trinket.io's Python 3.9.6 as well.
contextlib.nested supports this:
import contextlib
with contextlib.nested(open("out.txt","wt"), open("in.txt")) as (file_out, file_in):
...
Update:
To quote the documentation, regarding contextlib.nested:
Deprecated since version 2.7: The with-statement now supports this functionality directly (without the confusing error prone quirks).
See Rafał Dowgird's answer for more information.
nested context manager is a mistake and should never be used. In this example, if opening the second file raises an exception, the first file won't be closed at all, thus totally destroying the purpose of using context managers. –
Galangal with blocks. The managers are created in order before entering the with blocks: m1, m2, m3 = A(), B(), C() If B() or C() fails with exception, then your only hope of properly finalizing A() is the garbage collector. –
Galangal return and continue. –
Impermeable Since Python 3.3, you can use the class ExitStack from the contextlib module.
It can manage a dynamic number of context-aware objects, which means that it will prove especially useful if you don't know how many files you are going to handle.
The canonical use-case that is mentioned in the documentation is managing a dynamic number of files.
with ExitStack() as stack:
files = [stack.enter_context(open(fname)) for fname in filenames]
# All opened files will automatically be closed at the end of
# the with statement, even if attempts to open files later
# in the list raise an exception
Here is a generic example:
from contextlib import ExitStack
class X:
num = 1
def __init__(self):
self.num = X.num
X.num += 1
def __repr__(self):
cls = type(self)
return '{cls.__name__}{self.num}'.format(cls=cls, self=self)
def __enter__(self):
print('enter {!r}'.format(self))
return self.num
def __exit__(self, exc_type, exc_value, traceback):
print('exit {!r}'.format(self))
return True
xs = [X() for _ in range(3)]
with ExitStack() as stack:
print(stack._exit_callbacks)
nums = [stack.enter_context(x) for x in xs]
print(stack._exit_callbacks)
print(stack._exit_callbacks)
print(nums)
Output:
deque([])
enter X1
enter X2
enter X3
deque([<function ExitStack._push_cm_exit.<locals>._exit_wrapper at 0x7f5c95f86158>, <function ExitStack._push_cm_exit.<locals>._exit_wrapper at 0x7f5c95f861e0>, <function ExitStack._push_cm_exit.<locals>._exit_wrapper at 0x7f5c95f86268>])
exit X3
exit X2
exit X1
deque([])
[1, 2, 3]
I think you want to do this instead:
from __future__ import with_statement
with open("out.txt","wt") as file_out:
with open("in.txt") as file_in:
for line in file_in:
file_out.write(line)
From Python 3.10 there is a new feature of Parenthesized context managers, which permits syntax such as:
with (
A() as a,
B() as b
):
do_something(a, b)
In Python 3.1+ you can specify multiple context expressions, and they will be processed as if multiple with statements were nested:
with A() as a, B() as b:
suite
is equivalent to
with A() as a:
with B() as b:
suite
This also means that you can use the alias from the first expression in the second (useful when working with db connections/cursors):
with get_conn() as conn, conn.cursor() as cursor:
cursor.execute(sql)
You can also separate creating a context manager (the __init__ method) and entering the context (the __enter__ method) to increase readability. So instead of writing this code:
with Company(name, id) as company, Person(name, age, gender) as person, Vehicle(brand) as vehicle:
pass
you can write this code:
company = Company(name, id)
person = Person(name, age, gender)
vehicle = Vehicle(brand)
with company, person, vehicle:
pass
Note that creating the context manager outside of the with statement makes an impression that the created object can also be further used outside of the statement. If this is not true for your context manager, the false impression may counterpart the readability attempt.
The documentation says:
Most context managers are written in a way that means they can only be used effectively in a with statement once. These single use context managers must be created afresh each time they’re used - attempting to use them a second time will trigger an exception or otherwise not work correctly.
This common limitation means that it is generally advisable to create context managers directly in the header of the with statement where they are used.
d = Document('foo'); with d: d.write('bar'); print('OK', d.path) and I wasn't aware of this detail. –
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with open('./file') as arg.x = file:? – Eberhardtcontextlib.ExitStack. – Elegancy