Edit package installed by pip

Question

I'm trying to edit a package that I installed via pip, called py_mysql2pgsql (I had an error when converting my db from mysql to postgre, just like this.

However, when I got to the folder /user/local/lib/python2.7/dist-packages/py_mysql2pgsql-0.1.5.egg-info, I cannot find the source code for the package. I only find PKG-INFO and text files.

How can I find the actual source code for a package (or in particular, this package)?

Thanks

Answer 1

TL;DR:

Modifying in place is dangerous. Modify the source and then install it from your modified version.

Details

pip is a tool for managing the installation of packages. You should not modify files creating during package installation. At best, doing so would mean pip will believe that a particular version of the package is installed when it isn't. This would not interact well with the upgrade function. I suspect pip would just overwrite your customizations, discarding them forever, but I haven't confirmed. The other possibility is that it checks if files have changed and throws an error if so. (I don't think that's likely.) It also misleads other users of the system. They see that you have a package installed, but you don't actually have that version indicated; you have a customized version. This is likely to result in confusion if they try to install the unmodified version somewhere else or if they expect some particular behavior from the version installed.

If you want to modify the source code, the right thing to do is modify the source code and either build a new, custom package or just install from source. py-mysql2pgsql provides instructions for performing a source install:

> git clone git://github.com/philipsoutham/py-mysql2pgsql.git
> cd py-mysql2pgsql
> python setup.py install

You can clone the source, modify it, and then install without using pip. You could alternatively build your own customized version of the package if you need to redistribute it internally. This project uses setuptools for building its packages, so you only need to familiarize yourself with setuptools to make use of their setup.py file. Make sure that installing it this way doesn't create any misleading entries in pip's package list. If it does, either find a way to make sure the entry is more clear or find an alternative install method.

Since you've discovered a bug in the software, I also highly recommend forking it on Github and submitting a pull request once you have it fixed. If you do so, you can use the above installation instructions just by changing the repository URL to your fork. If you don't fork it, at least file an issue and describe the changes that fix it.

Alternatives:

• You could copy all the source code into your project, modify it there, and then distribute the modified version with the rest of your code. (Make sure you don't violate the license if you do so.)
• You might be able to solve you problem at runtime. Monkey-patching the module is a little risky if other people on your team might not expect the change in behavior, but it could be done for global modification of the module's behavior. You could also create some additional code that wraps the buggy code: it can take input, call the buggy code, and either prevents or handles the bug (e.g., modifying the input to make it work or catching an exception and handling it, etc.).