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Python Compiler Internals Thomas Lee Shine Technologies, Melbourne

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My Contributions to Python <ul><li>try/except/finally syntax for 2.5 </li></ul><ul><li>AST to bytecode compilation from Python </li></ul><ul><li>“ optimization” at the AST level </li></ul>

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What will you get out of this? Compiler development != rocket science. No magic: it's just code.

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Compiler? Isn't Python interpreted? Well, yes and no.

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WTF It ... It's Java?

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Aaanyway Let's screw with the compiler.

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Erm ... Before we start ... A patch to fix a quirk in the build process. This has been reported on the bug tracker!

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An overview of the compiler Tokens Parse Tree AST Bytecode + Data AST + Symtable Tokenizer Parser AST Translation Bytecode Generation Bytecode Optimization Execution Symtable Construction

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New construct: the “unless” statement unless may_give_you_up: print “never gonna give you up...”

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Semantics of “unless” Works just like “if not” ... if not would_consider_letting_you_down: print “never gonna let you down...”

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Implementing “unless” <ul><li>Modify the Grammar </li></ul><ul><li>Change the AST definition </li></ul><ul><li>Generate bytecode for “unless” </li></ul>

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So what's first? <ul><li>Modify the Grammar </li></ul><ul><li>Change the AST definition </li></ul><ul><li>Generate bytecode for “unless” </li></ul>

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What does this change affect? Tokens Parse Tree AST Bytecode + Data AST + Symtable Tokenizer Parser AST Translation Bytecode Generation Bytecode Optimization Execution Symtable Construction

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WTF is a “grammar”? Defines the syntactic structure of the language.

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Why a tokenizer? Makes parsing easier.

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So what does a tokenizer do, then? Generates a stream of events for the parser.

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What does the parser do? Organises tokens into the structure defined by the grammar. This is the “parse tree”.

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What's next? <ul><li>Modify the Grammar </li></ul><ul><li>Change the AST definition </li></ul><ul><li>Generate bytecode for “unless” </li></ul>

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What's affected by this change? Tokens Parse Tree AST Bytecode + Data AST + Symtable Tokenizer Parser AST Translation Bytecode Generation Bytecode Optimization Execution Symtable Construction

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What's an AST? Similar to a parse tree, but not bound to the syntax “ Abstract Syntax Tree”

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Why does Python use an AST? Much easier to operate upon an AST than a parse tree.

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AST reuse New constructs can sometimes be expressed using existing AST nodes ...

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AST reuse ... so we could implement “unless” using “if” and “not” AST nodes. But then I wouldn't be able to show you the code generator ... Unless (test, body) = If ( Not (test), body)

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What's next? <ul><li>Modify the Grammar </li></ul><ul><li>Change the AST definition </li></ul><ul><li>Generate bytecode for “unless” </li></ul>

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What's affected by this change? Tokens Parse Tree AST Bytecode + Data AST + Symtable Tokenizer Parser AST Translation Bytecode Generation Bytecode Optimization Execution Symtable Construction

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Generating bytecode for “unless” <ul><li>Add a hook for our new AST node </li></ul><ul><li>Generate bytecode implementing “unless” logic </li></ul><ul><li>Use basicblocks as labels for jumps </li></ul>

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What about the rest of the compiler? Tokens Parse Tree AST Bytecode + Data AST + Symtable Tokenizer Parser AST Translation Bytecode Generation Bytecode Optimization Execution Symtable Construction

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“unless” in action Let's see if this stuff works ...

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Where to go from here? <ul><li>Use the Source. </li></ul><ul><li>Don't try to grok it all at once. </li></ul><ul><li>Don't be afraid to be wrong. </li></ul><ul><li>Annoy the python-dev mailing list, like I do! </li></ul>

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Thanks!

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Questions?