Coding guide — IPython v0.10 documentation

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# Coding guide

## Coding conventions

In general, we’ll try to follow the standard Python style conventions as described in Python’s PEP 8, the official Python Style Guide.

Other comments:

- In a large file, top level classes and functions should be separated by 2-3 lines to make it easier to separate them visually.
- Use 4 spaces for indentation, never use hard tabs.
- Keep the ordering of methods the same in classes that have the same methods. This is particularly true for classes that implement similar interfaces and for interfaces that are similar.

### Naming conventions

In terms of naming conventions, we’ll follow the guidelines of PEP 8. Some of the existing code doesn’t honor this perfectly, but for all new IPython code (and much existing code is being refactored), we’ll use:

- All `lowercase` module names.
- `CamelCase` for class names.
- `lowercase_with_underscores` for methods, functions, variables and attributes.

This may be confusing as some of the existing codebase uses a different convention (`lowerCamelCase` for methods and attributes). Slowly, we will move IPython over to the new convention, providing shadow names for backward compatibility in public interfaces.

There are, however, some important exceptions to these rules. In some cases, IPython code will interface with packages (Twisted, Wx, Qt) that use other conventions. At some level this makes it impossible to adhere to our own standards at all times. In particular, when subclassing classes that use other naming conventions, you must follow their naming conventions. To deal with cases like this, we propose the following policy:

- If you are subclassing a class that uses different conventions, use its naming conventions throughout your subclass. Thus, if you are creating a Twisted Protocol class, used Twisted’s `namingSchemeForMethodsAndAttributes.`
- All IPython’s official interfaces should use our conventions. In some cases this will mean that you need to provide shadow names (first implement `fooBar` and then `foo_bar = fooBar`). We want to avoid this at all costs, but it will probably be necessary at times. But, please use this sparingly!

Implementation-specific private methods will use `_single_underscore_prefix`. Names with a leading double underscore will only be used in special cases, as they makes subclassing difficult (such names are not easily seen by child classes).

Occasionally some run-in lowercase names are used, but mostly for very short names or where we are implementing methods very similar to existing ones in a base class (like `runlines()` where `runsource()` and `runcode()` had established precedent).

The old IPython codebase has a big mix of classes and modules prefixed with an explicit `IP`. In Python this is mostly unnecessary, redundant and frowned upon, as namespaces offer cleaner prefixing. The only case where this approach is justified is for classes which are expected to be imported into external namespaces and a very generic name (like Shell) is too likely to clash with something else. We’ll need to revisit this issue as we clean up and refactor the code, but in general we should remove as many unnecessary `IP`/`ip` prefixes as possible. However, if a prefix seems absolutely necessary the more specific `IPY` or `ipy` are preferred.

## Testing system

It is extremely important that all code contributed to IPython has tests. Tests should be written as unittests, doctests or as entities that the Nose testing package will find. Regardless of how the tests are written, we will use Nose for discovering and running the tests. Nose will be required to run the IPython test suite, but will not be required to simply use IPython.

Tests of Twisted using code should be written by subclassing the `TestCase` class that comes with `twisted.trial.unittest`. When this is done, Nose will be able to run the tests and the twisted reactor will be handled correctly.

Each subpackage in IPython should have its own `tests` directory that contains all of the tests for that subpackage. This allows each subpackage to be self-contained. If a subpackage has any dependencies beyond the Python standard library, the tests for that subpackage should be skipped if the dependencies are not found. This is very important so users don’t get tests failing simply because they don’t have dependencies.

We also need to look into use Noses ability to tag tests to allow a more modular approach of running tests.

## Configuration system

IPython uses .ini files for configuration purposes. This represents a huge improvement over the configuration system used in IPython. IPython works with these files using the ConfigObj package, which IPython includes as `ipython1/external/configobj.py`.

Currently, we are using raw ConfigObj objects themselves. Each subpackage of IPython should contain a `config` subdirectory that contains all of the configuration information for the subpackage. To see how configuration information is defined (along with defaults) see at the examples in `ipython1/kernel/config` and `ipython1/core/config`. Likewise, to see how the configuration information is used, see examples in `ipython1/kernel/scripts/ipengine.py`.

Eventually, we will add a new layer on top of the raw ConfigObj objects. We are calling this new layer, `tconfig`, as it will use a Traits-like validation model. We won’t actually use Traits, but will implement something similar in pure Python. But, even in this new system, we will still use ConfigObj and .ini files underneath the hood. Talk to Fernando if you are interested in working on this part of IPython. The current prototype of `tconfig` is located in the IPython sandbox.

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