Jedi Development

Note

This documentation is for Jedi developers who want to improve Jedi itself, but have no idea how Jedi works. If you want to use Jedi for your IDE, look at the plugin api.

Introduction

This page tries to address the fundamental demand for documentation of the Jedi internals. Understanding a dynamic language is a complex task. Especially because type inference in Python can be a very recursive task. Therefore Jedi couldn’t get rid of complexity. I know that simple is better than complex, but unfortunately it sometimes requires complex solutions to understand complex systems.

Since most of the Jedi internals have been written by me (David Halter), this introduction will be written mostly by me, because no one else understands to the same level how Jedi works. Actually this is also the reason for exactly this part of the documentation. To make multiple people able to edit the Jedi core.

In five chapters I’m trying to describe the internals of Jedi:

Note

Testing is not documented here, you’ll find that right here.

The Jedi Core

The core of Jedi consists of three parts:

Most people are probably interested in code evaluation, because that’s where all the magic happens. I need to introduce the parser first, because jedi.evaluate uses it extensively.

Parser (parser/__init__.py)

Parser Tree (parser/tree.py)

Class inheritance diagram:

Evaluation of python code (evaluate/__init__.py)

Evaluation of Python code in Jedi is based on three assumptions:

  • The code uses as least side effects as possible. Jedi understands certain list/tuple/set modifications, but there’s no guarantee that Jedi detects everything (list.append in different modules for example).
  • No magic is being used:
    • metaclasses
    • setattr() / __import__()
    • writing to globals(), locals(), object.__dict__
  • The programmer is not a total dick, e.g. like this :-)

The actual algorithm is based on a principle called lazy evaluation. That said, the typical entry point for static analysis is calling eval_expr_stmt. There’s separate logic for autocompletion in the API, the evaluator is all about evaluating an expression.

TODO this paragraph is not what jedi does anymore.

Now you need to understand what follows after eval_expr_stmt. Let’s make an example:

import datetime
datetime.date.toda# <-- cursor here

First of all, this module doesn’t care about completion. It really just cares about datetime.date. At the end of the procedure eval_expr_stmt will return the date class.

To visualize this (simplified):

  • Evaluator.eval_expr_stmt doesn’t do much, because there’s no assignment.
  • Context.eval_node cares for resolving the dotted path
  • Evaluator.find_types searches for global definitions of datetime, which it finds in the definition of an import, by scanning the syntax tree.
  • Using the import logic, the datetime module is found.
  • Now find_types is called again by eval_node to find date inside the datetime module.

Now what would happen if we wanted datetime.date.foo.bar? Two more calls to find_types. However the second call would be ignored, because the first one would return nothing (there’s no foo attribute in date).

What if the import would contain another ExprStmt like this:

from foo import bar
Date = bar.baz

Well... You get it. Just another eval_expr_stmt recursion. It’s really easy. Python can obviously get way more complicated then this. To understand tuple assignments, list comprehensions and everything else, a lot more code had to be written.

Jedi has been tested very well, so you can just start modifying code. It’s best to write your own test first for your “new” feature. Don’t be scared of breaking stuff. As long as the tests pass, you’re most likely to be fine.

I need to mention now that lazy evaluation is really good because it only evaluates what needs to be evaluated. All the statements and modules that are not used are just being ignored.

Evaluation Representation (evaluate/representation.py)

Name resolution (evaluate/finder.py)

Searching for names with given scope and name. This is very central in Jedi and Python. The name resolution is quite complicated with descripter, __getattribute__, __getattr__, global, etc.

If you want to understand name resolution, please read the first few chapters in http://blog.ionelmc.ro/2015/02/09/understanding-python-metaclasses/.

Flow checks

Flow checks are not really mature. There’s only a check for isinstance. It would check whether a flow has the form of if isinstance(a, type_or_tuple). Unfortunately every other thing is being ignored (e.g. a == ‘’ would be easy to check for -> a is a string). There’s big potential in these checks.

API (api.py and api_classes.py)

The API has been designed to be as easy to use as possible. The API documentation can be found here. The API itself contains little code that needs to be mentioned here. Generally I’m trying to be conservative with the API. I’d rather not add new API features if they are not necessary, because it’s much harder to deprecate stuff than to add it later.

Core Extensions

Core Extensions is a summary of the following topics:

These topics are very important to understand what Jedi additionally does, but they could be removed from Jedi and Jedi would still work. But slower and without some features.

Iterables & Dynamic Arrays (evaluate/iterable.py)

To understand Python on a deeper level, Jedi needs to understand some of the dynamic features of Python like lists that are filled after creation:

Parameter completion (evaluate/dynamic.py)

One of the really important features of Jedi is to have an option to understand code like this:

def foo(bar):
    bar. # completion here
foo(1)

There’s no doubt wheter bar is an int or not, but if there’s also a call like foo('str'), what would happen? Well, we’ll just show both. Because that’s what a human would expect.

It works as follows:

  • Jedi sees a param
  • search for function calls named foo
  • execute these calls and check the input.

Diff Parser (parser/diff.py)

Docstrings (evaluate/docstrings.py)

Docstrings are another source of information for functions and classes. jedi.evaluate.dynamic tries to find all executions of functions, while the docstring parsing is much easier. There are three different types of docstrings that Jedi understands:

For example, the sphinx annotation :type foo: str clearly states that the type of foo is str.

As an addition to parameter searching, this module also provides return annotations.

Refactoring (evaluate/refactoring.py)

Introduce some basic refactoring functions to Jedi. This module is still in a very early development stage and needs much testing and improvement.

Warning

I won’t do too much here, but if anyone wants to step in, please do. Refactoring is none of my priorities

It uses the Jedi API and supports currently the following functions (sometimes bug-prone):

  • rename
  • extract variable
  • inline variable

Imports & Modules

Compiled Modules (evaluate/compiled.py)

Imitate the parser representation.

Imports (evaluate/imports.py)

jedi.evaluate.imports is here to resolve import statements and return the modules/classes/functions/whatever, which they stand for. However there’s not any actual importing done. This module is about finding modules in the filesystem. This can be quite tricky sometimes, because Python imports are not always that simple.

This module uses imp for python up to 3.2 and importlib for python 3.3 on; the correct implementation is delegated to _compatibility.

This module also supports import autocompletion, which means to complete statements like from datetim (curser at the end would return datetime).

Caching & Recursions

Caching (cache.py)

This caching is very important for speed and memory optimizations. There’s nothing really spectacular, just some decorators. The following cache types are available:

  • time_cache can be used to cache something for just a limited time span, which can be useful if there’s user interaction and the user cannot react faster than a certain time.

This module is one of the reasons why Jedi is not thread-safe. As you can see there are global variables, which are holding the cache information. Some of these variables are being cleaned after every API usage.

Recursions (recursion.py)

Recursions are the recipe of Jedi to conquer Python code. However, someone must stop recursions going mad. Some settings are here to make Jedi stop at the right time. You can read more about them here.

Next to jedi.evaluate.cache this module also makes Jedi not thread-safe. Why? execution_recursion_decorator uses class variables to count the function calls.

Settings

Recursion settings are important if you don’t want extremly recursive python code to go absolutely crazy.

The default values are based on experiments while completing the Jedi library itself (inception!). But I don’t think there’s any other Python library that uses recursion in a similarly extreme way. Completion should also be fast and therefore the quality might not always be maximal.

jedi.evaluate.recursion.recursion_limit = 15

Like sys.getrecursionlimit(), just for Jedi.

jedi.evaluate.recursion.total_function_execution_limit = 200

This is a hard limit of how many non-builtin functions can be executed.

jedi.evaluate.recursion.per_function_execution_limit = 6

The maximal amount of times a specific function may be executed.

jedi.evaluate.recursion.per_function_recursion_limit = 2

A function may not be executed more than this number of times recursively.

Helper Modules

Most other modules are not really central to how Jedi works. They all contain relevant code, but you if you understand the modules above, you pretty much understand Jedi.

Python 2/3 compatibility (_compatibility.py)

To ensure compatibility from Python 2.6 - 3.3, a module has been created. Clearly there is huge need to use conforming syntax.