# Developer’s guide¶

## Python code standards¶

### Code formatting style¶

The formatting style is black with a line length of 89, which is specified in the .flake8 file that is part of the source repository.

The beauty of formatting with black is that you can just write all your Python code and then format at the end. For example, with vim or neovim, you may use the plugin from the black repository and the following normal mode command to auto-format your code:

:Black


Note

At the time of this writing (April 11, 2020), vim/nvim integration with black requires installing black from the GitHub repo and not from PyPi or conda.

pip3 install git+git://github.com/psf/black


Placing the following in your vim or neovim configuration file will map this command to the F6 button:

autocmd FileType python nnoremap <buffer> <f6> :Black<CR>


The black repository describes methods for integration with other editors.

Note

Other than line length, all black parameters used are the defaults!

In addition to black, import statements should be sorted using isort:

isort file.py


Or, in vim/neovim:

:!isort %


Note

In the future, we may rely entirely on black to sort includes as black/isort compatibility evolves.

## Writing documentation¶

### Formatting the Python code in rst files¶

Code blocks in documentation should be formatted using blacken-docs. In general, you can write your code blocks and format them after the fact using the following command:

blacken-docs -E -l 89 -t py36 file.rst


Using an editor like vim or neovim, you can format from within the editor using the following normal mode command:

:!blacken-docs -E -l 89 -t py36 %


Code blocks written using the iPython directive may fail to execute after formatting. Usually, this is due to blank likes being added in order to generate PEP8-compliant Python code. However, this procedure often generates too many blank lines and the iPython parser raises an exception. Sigh. To work around this:

• Add more .. ipython:: python directives to split the blocks up

• Wrap class definitions in # fmt: off and # fmt: on comments to disable formatting using black. You can find examples by grepping for fmt within the .rst files.

The latter recommendation is needed because PEP8 wants blank lines between the definitions of class functions, yet the iPython parser will fail to properly parse such a class. I tend to run blacken-docs after defining a class, manually delete the blank lines, and then wrap in the fmt comments. This procedure is manual but not too burdensome.