# [FRPythoneers] a more generic sort

jason songhurst jason.songhurst at valtech.com
Tue Jan 21 12:30:33 MST 2003

```The optional argument of the sort() method must be either:
1) a function of exactly two non optional arguments
2) an object that implements the '__call__(self, [arg]*)' method with
two non-optional arguments.

Also, the function or function-like object must return a number.  But
you weren't having problems there...  Yet.

Andrew Diederich wrote:
> def mySort2(a, b, param):
>         return cmp(a[param], b[param])
>
> res.sort(mySort2(a, b, 'cn'))

This wrong for a least two reasons.  First, you are calling mySort with
a non-existent variable.  Second, you are trying to pass the results of
your sorting function (a number) to the sort() method.  It can't take a
number, it only takes functions. So we can rule out this approach, even
without the error.

> def mySort3(a, b, param):
>         return cmp(a[param], b[param])
>
> res.sort(mySort3('cn'))

This is wrong for the same reason as the first.  mySort3 takes three
parameters, and returns a number. So you are passing mySort3() one
parameter (wrong), and sending (well, trying to send) the numeric result
to sort() which only takes functions (that's wrong).  So this approach
is out as well.

> res.sort( lambda x, y, param: cmp(x[param], y[param]) )

Again, the function passed to sort() must take exactly two parameters,
one is no good and three is quite out.

I think Jack already said this, but you could do:

column = 'cn'
res.sort(lambda x, y, param=column: cmp(x[param], y[param]))

which creates an anonymous function of three parameters, and makes the
third parameter 'p' optional, with a defaults value set to the value of
the variable 'param'.

or you could try something along the lines of:

class compareColumn:
def __init__(self, param):
self.param = param
def __call__(self, a, b):
return cmp(a[self.param], b[self.param])

res.sort(compareColumn('cn'))

Which creates an object that looks like a function taking two parameters
to pass into sort.

Good luck,

Jason Songhurst

```

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