PyCharm can leverage run-time information when running your application with the built-in debugger to figure out what types can possibly be passed to which functions, etc.

PyCharm, based on what method or class the cursor rests, can figure out what tests to run and perform them with a keyboard shortcut or two, without breaking up the flow and need to switch to a command line interface.

There are many refactoring options including renaming and changing signature across entire projects. It also includes the an ability to preview changes before committing and exclude anything unwanted.

All the debugging can be done inside the IDE. Breakpoints in the code can be added using keyboard shortcuts or the mouse. When the code is executed through the debugger a toolbar pops up with all the relevant context needed for the debugging process.

The whole process is smooth and painless and you don't even have to switch windows to do the debugging.

PyCharm has two types of autocompletion: structural completion and word expansion.

Structural autocompletion makes predictions based on its understanding of Python and JavaScript objects, while the latter tries to predict the word currently being typed based on previously typed words. Word expansion also works in comments and docstrings and it's similar to vim's omnicompletion.

Both types of autocompletion work extremely well, have little to no problems and are quite fast even when loading suggestions on the go.

There's a community edition (with limited features) that's free to use. You can also get a 30-day trial of the Professional edition.

PyCharm has amazing code navigation implementations. It supports both goto symbol and goto declaration. The former finds classes, variables, functions, etc by name. While the latter is used by moving the cursor on top of a symbol and by using the mouse or a keyboard combination it finds the declaration of that symbol and takes you there.

Both of these features are extremely helpful when consulting large code-bases and when trying to understand an API written by someone else.

PyCharm has CVS, Git, Subversion and Mercurial integration.

PyCharm offers great pip integration. When opening a project it automatically checks for a requirements.txt file in the root of the project. If it's found, it checks if all the libraries are available in the interpreter. If one or more libraries are missing, it issues a warning and asks whether you want to install any missing libraries.