Pass variable by reference
Terry Reedy
tjreedy at udel.edu
Tue May 6 15:16:54 EDT 2014
On 5/5/2014 8:39 PM, Satish Muthali wrote:
> Hello experts,
>
> I have a burning question on how to pass variable by reference in
> Python.
Python passes objects to functions. CPython implements this by passing
object pointers. In one sense, your request is impossible. In another,
it already happens.
> I understand that the data type has to be mutable.
In C, all data are mutable, so copying a block of memory versus passing
a pointer is the meaningful distinction. In Python, mutable arguments
can be mutated and immutable args cannot, so *that* is the only
meaningful distinction. Strings are not mutable.
> For example, here’s the issue I am running in to:
>
> I am trying to extract the PostgreSQL DB version for example:
>
> /pgVer = [s.split() for s in os.popen("psql
> --version").read().splitlines()]/
> / print pgVer[0]/
> / for i, var in enumerate(pgVer[0]):/
> / if i == len(pgVer[0]) - 1:/
> / pgversion = var/
> /
> /
> I would now like to pass ‘pgversion’ (where the value of pgversion is
> 9.3.4) by reference, for example:
>
> I want to nuke /var/lib/postgresql/9.3.4/main/data , however
> programatically I want it to be as: /var/lib/postgresql/*/<value of
> pgversion>/*/main/data
It is not clear whether you want to delete a file or rename it. Either
is easy and neither depend on passing mechanism.
--
Terry Jan Reedy
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