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README.rst

YAPF

Introduction

Most of the current formatters for Python -- e.g., autopep8, and pep8ify -- are made to remove lint errors from code. This has some obvious limitations. For instance, code that conforms to the PEP 8 guidelines may not be reformatted. But it doesn't mean that the code looks good.

YAPF takes a different approach. It's based off of 'clang-format', developed by Daniel Jasper. In essence, the algorithm takes the code and reformats it to the best formatting that conforms to the style guide, even if the original code didn't violate the style guide.

The ultimate goal is that the code YAPF produces is as good as the code that a programmer would write if they were following the style guide.

Installation

From source directory:

$ sudo python ./setup.py install

Usage

<verbatim> usage: __main__.py [-h] [-d | -i] [-l START-END | -r] ...

Formatter for Python code.

positional arguments:

files

optional arguments:

-h, --help show this help message and exit -d, --diff print the diff for the fixed source -i, --in-place make changes to files in place -l START-END, --lines START-END range of lines to reformat, one-based -r, --recursive run recursively over directories

</verbatim>

Why Not Improve Existing Tools?

We wanted to use clang-format's reformatting algorithm. It's very powerful and designed to come up with the best formatting possible. Existing tools were created with different goals in mind, and would require extensive modifications to convert to using clang-format's algorithm.

Can I Use YAPF In My Program?

Please do! YAPF was designed to be used as a library as well as a command line tool. This means that a tool or IDE plugin is free to use YAPF.

Gory Details

Algorithm Design

The main data structure in YAPF is the UnwrappedLine object. It holds a list of FormatTokens, that we would want to place on a single line if there were no column limit. An exception being a comment in the middle of an expression statement will force the line to be formatted on more than one line. The formatter works on one UnwrappedLine object at a time.

An UnwrappedLine typically won't affect the formatting of lines before or after it. There is a part of the algorithm that may join two or more UnwrappedLines into one line. For instance, an if-then statement with a short body can be placed on a single line:

if a == 42: continue

YAPF's formatting algorithm creates a weighted tree that acts as the solution space for the algorithm. Each node in the tree represents the result of a formatting decision --- i.e., whether to split or not to split before a token. Each formatting decision has a cost associated with it. Therefore, the cost is realized on the edge between two nodes. (In reality, the weighted tree doesn't have separate edge objects, so the cost resides on the nodes themselves.)

For example, take the following Python code snippet. For the sake of this example, assume that line (1) violates the column limit restriction and needs to be reformatted.

1: def xxxxxxxxxxx(aaaaaaaaaaaa, bbbbbbbbb, cccccccc, dddddddd, eeeeee): 2: pass

For line (1), the algorithm will build a tree where each node (a FormattingDecisionState object) is the state of the line at that token given the decision to split before the token or not. Note: the FormatDecisionState objects are copied by value so each node in the graph is unique and a change in one doesn't affect other nodes.

Here is a hypothetical subtree of the first five tokens. The value in parentheses is the hypothetical cost of splitting before the token. (The left hand branch is a decision to split and the right hand branch is a decision not to split.)

'def'

 (0)
'xxxxxxxxxxx'

 (0)

'(' |

(3) +---------------------------+ (1) | | | | 'aaaaaaaaaaaa' 'aaaaaaaaaaaa' | | | | +--------------+ +-------------+ | | | | (50) | (0) | (50) | (0) | ',' ',' ',' ','

And so on. Heuristics are used to determine the costs of splitting or not splitting. Because a node holds the state of the tree up to a token's insertion, it can easily determine if a splitting decision will violate one of the style requirements. For instance, the heuristic is able to apply an extra penalty to the edge when not splitting between the previous token and the one being added.

There are some instances where we will never want to split the line, because doing so will always be detrimental (i.e., it will require a backslash-newline, which is very rarely desirable). For line (1), we will never want to split the first three tokens: 'def', 'xxxxxxxxxxx', and '('. Nor will we want to split between the ')' and the ':' at the end. These regions are said to be "unbreakable." This is reflected in the tree by there not being a 'split' decision (left hand branch) within the unbreakable region.

Now that we have the tree, we determine what the "best" formatting is by finding the path through the tree with the lowest cost.

And that's it!