Refactoring Code
With the Standard Library
John Reese
Production Engineer, Facebook
@n7cmdr
github.com/jreese
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• Principal sponsor of the PSF
• Third most popular in Facebook
• Biggest language in Instagram
• Frontend, backend, tools, and automation
• Need safe, reliable refactoring tools
Python @ Facebook
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• Modify source code
• Change names or interfaces
• Update all references
Refactoring
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• Consistent style or formatting
• Remove code smells
• Enhance or replace an API
• Support new use cases
• Remove dead code
Why refactor?
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• Usually automated refactoring
• Atomic changes to the entire codebase
• Update API and consumers simultaneously
• Ensure no build/tests are broken
Code mods
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Syntax Trees
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• Modify code as nested objects
• Based on Python grammar
• Semantic context for elements
• “Guaranteed” valid syntax
Syntax tree refactoring
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• Tree structure, nodes and leaves
• Semantic representation of code
• Intended for execution
Abstract syntax tree
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Call
Name []
args
[]
func keywords
print
id
Str
‘Hello World’
s
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• Tree structure, nodes and leaves
• Decomposed units of syntax and grammar
• Literal representation of on-disk code
• Whitespace, formatting, comments, etc
Concrete syntax tree
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lib2to3
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• Concrete syntax tree
• Built for the 2to3 tool
• Can parse all Python grammars
lib2to3
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• Part of the standard library
• Always up to date with new syntax
• Contains refactoring framework
Why lib2to3?
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• Leaf for each distinct token
• Node for semantic groupings
• Nodes contain one or more children
• Generic objects, token/symbol type
• Collapsed grammar
Tree structure
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power
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Building Code Mods
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• Designed for 2to3 tools
• Pattern match to find elements
• In-place transforms to tree
Fixers
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• Search for grammar elements
• Can be arbitrarily nested, combined
• Capture specific nodes or leaves
• Include literals or token types
Pattern Matching
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• Called for each match
• Add, modify, remove, or replace elements
• Not restricted to matched elements
Transforms
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• Runs fixers on each file
• Runs transforms at matching nodes
• Collects final tree to diff/write
• Defaults to loading 2to3 fixers
Refactoring Tool
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github.com/jreese/pycon
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Safe refactoring for modern Python
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• Code mod framework
• Built on lib2to3 primitives
• Fluent API to generate fixers
• Optimized for large codebases
• MIT Licensed
Bowler
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• Automatic support for new Python releases
• Encourages reuse of components
• Productionizes common refactoring
• Useful as a tool and a library
Why Bowler?
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• Selectors build a search pattern
• Optionally filter elements
• Modify matched elements
• Compose multiple transforms
• Generate diffs or interactive results
Query pipeline
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Query Chaining
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• Facebook Incubator project
• Fluent API is fluid
• Incomplete set of selectors, filters, transforms
• Needs more unit testing
Bowler
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• More use cases
• Linter features
• Integrations
• More testing
• More contributors!
Roadmap
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https://pybowler.io
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John Reese
Production Engineer, Facebook
@n7cmdr
github.com/jreese
https://pybowler.io