I have been working on a Python extension module with lots of files. While building on one machine, python setup.py build will happily detect changed files, build just those files, and link the whole thing together, just like make. On another machine, however, a single change to any file triggers a recompile of all sources.
Just to be clear. Both machines detect when the package is up to date and won’t do anything. It is only when a single file changes that their behavior diverges.
Why is the second machine doing this?
Machine 1 (Does proper per-file dependency check and build.)
Python 2.6.4 (r264:75706, Feb 15 2010, 17:06:03)
[GCC 4.1.2 20080704 (Red Hat 4.1.2-44)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
setuptools-0.6c11-py2.6
LSB Version: :core-3.1-amd64:core-3.1-ia32:core-3.1-noarch:graphics-3.1-amd64:graphics-3.1-ia32:graphics-3.1-noarch
Distributor ID: CentOS
Description: CentOS release 5.4 (Final)
Release: 5.4
Codename: Final
Machine 2 (Rebuilds everything when a single source file changes.)
Python 2.6.5 (r265:79063, Apr 16 2010, 13:57:41)
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
setuptools-0.6c11-py2.6
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 10.04 LTS
Release: 10.04
Codename: lucid
I looked into the Mercurial repo and found this change:
Issue #5372: Drop the reuse of .o files in Distutils’ ccompiler (since Extension extra options may change the output without changing the .c file).
IOW, it was a simplistic optimization that was removed.