Multiple variables in a with statement

Question

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

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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

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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

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Note that if you split the variables into lines  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   When will this be fixed  This issue is tracked in https   bugs python org issue12782  Recently  Python announced in PEP 617 that they ll be replacing the current parser with a new one  Because Python s current parser is LL 1   it cannot distinguish between  quot multiple context managers quot  with  A    B     and  quot tuple of values quot  with  A    B    0    The new parser can properly parse  quot multiple context managers quot  surrounded by tuples  The new parser will be enabled in 3 9  but this syntax will still be rejected until the old parser is removed in Python 3 10

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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 Rafal Dowgird s answer for more information

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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

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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

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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   lt function ExitStack  push cm exit  lt locals gt   exit wrapper at 0x7f5c95f86158 gt    lt function ExitStack  push cm exit  lt locals gt   exit wrapper at 0x7f5c95f861e0 gt    lt function ExitStack  push cm exit  lt locals gt   exit wrapper at 0x7f5c95f86268 gt    exit X3 exit X2 exit X1 deque      1  2  3

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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

[python] Multiple variables in a 'with' statement?

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