# Programming Tutorials

## Schwartzian Transform in python

By: Python Documentation Team Printer Friendly Format

The technique, attributed to Randal Schwartz of the Perl community, sorts the elements of a list by a metric which maps each element to its "sort value". In Python, just use the key argument for the sort() method:

```Isorted = L[:] Isorted.sort(key=lambda s: int(s[10:15])) ```
The key argument is new in Python 2.4, for older versions this kind of sorting is quite simple to do with list comprehensions. To sort a list of strings by their uppercase values:

```tmp1 = [(x.upper(), x) for x in L]  # Schwartzian transform tmp1.sort() Usorted = [x for x in tmp1] ```
To sort by the integer value of a subfield extending from positions 10-15 in each string:

```tmp2 = [(int(s[10:15]), s) for s in L]  # Schwartzian transform tmp2.sort() Isorted = [x for x in tmp2] ```
For versions prior to 3.0, Isorted may also be computed by

``` def intfield(s):     return int(s[10:15]) def Icmp(s1, s2):     return cmp(intfield(s1), intfield(s2)) Isorted = L[:] Isorted.sort(Icmp) ```

but since this method calls intfield() many times for each element of L, it is slower than the Schwartzian Transform.

 Most Viewed Articles (in Python ) Latest Articles (in Python)

Comment on this tutorial