=============================================
Building Expressions from Python Syntax Trees
=============================================

The ``ast_builder`` module allows you to quickly navigate a Python syntax
tree and perform operations on it.  While Python 2.5 has a new "AST" feature
that provides a high-level syntax tree, older Python versions offer a very
low-level interface that provides complex tuple trees with lots of redundant
information.  The ``ast_builder`` module simplifies these trees dramatically,
without creating an intermediate AST data structure (the way the stdlib
``compiler`` package does).  Instead, it allows you to effectively "visit"
a virtual AST structure and generate your desired output directly.  In
addition, it allows you to skip, delay, or repeat traversals of arbitrary
subtrees.

This document describes the design (and tests the implementation) of the
``ast_builder`` module.  You don't need to read it unless you want to use
this module directly in your own programs.  If you do want to use it directly,
you should keep in mind that it currently only implements a **subset** of Python
*expression* syntax: it does not support lambdas, yield expressions, or any
kind of statements.

.. contents:: **Table of Contents**


------------------------
Parse Trees and Builders
------------------------

``ast_builder`` operates on parse tuple trees, as created by the standard
library ``parser`` module.  The two API functions it provides are ``build``
and ``parse_expr``::

    >>> from peak.rules.ast_builder import build, parse_expr

The ``build()`` function accepts two arguments, a "builder" and a "nodelist".
A "builder" is an object that you supply that will perform actions on nodes in
the parse tree.  The "nodelist" is a parse tuple tree.  As a shortcut, you
can use ``parse_expr()`` to parse a string into a nodelist and invoke
``build()`` in one step.

A simple example::

    >>> class Builder:
    ...     def Const(self, const):
    ...         print const

    >>> parse_expr("1", Builder())
    1

    >>> parse_expr("'foo'", Builder())
    foo

As you can see, the builder's ``Const()`` method is invoked for integer and
string constants, and it receives an actual value.  Many other method names
are used for more complex operations::

    >>> class Builder:
    ...     def Const(self, const):
    ...         return repr(const)
    ...     def Add(self, left, right):
    ...         return "Add(%s, %s)" % (build(self,left), build(self,right))

    >>> parse_expr("1+2", Builder())
    'Add(1, 2)'

    >>> parse_expr("'foo'+'bar'", Builder())
    "Add('foo', 'bar')"

Most builder methods accept nodelists as arguments.  These nodelists can be
recursively passed to ``build()`` in order to process expression subtrees.
This is not done automatically, because it's possible you might want to skip
processing of a particular subtree, or need to process a subtree with a
different builder than the one currently in use, or even process a subtree with
more than one builder (e.g. a builder that sees what names are bound within
a function body, and a second builder to generate code).

For convenience in the rest of this document, we'll use a shorthand function to
create a ``Builder()``, parse an expression, and print the result::

    >>> def pe(expr):
    ...     print parse_expr(expr, Builder())


Tokens
======

The ``Const`` and ``Name`` methods receive token values for constants and
names respectively::

    >>> class Builder:
    ...     def Const(self, const):
    ...         return repr(const)
    ...
    ...     def Name(self, name):
    ...         return name

    >>> pe("a")
    a
    >>> pe("b")
    b
    >>> pe("123")
    123
    >>> pe("'xyz'")
    'xyz'

Note that adjacent string constants are automatically merged::

    >>> pe("'abc' 'xyz'")
    'abcxyz'


Unary Operators
===============

There are five "unary" operator methods, that take a single argument: an AST
tuple representing the expression the operator is applied to::

    >>> class Builder(Builder):
    ...     def unaryOp(fmt):
    ...         def method(self,expr):
    ...             return fmt % build(self,expr)
    ...         return method
    ...
    ...     UnaryPlus  = unaryOp('Plus(%s)')
    ...     UnaryMinus = unaryOp('Minus(%s)')
    ...     Invert     = unaryOp('Invert(%s)')
    ...     Backquote  = unaryOp('repr(%s)')
    ...     Not        = unaryOp('Not(%s)')

    >>> pe("not - + ~`x`")
    Not(Minus(Plus(Invert(repr(x)))))


Attribute Access
================

The ``Getattr()`` method is called with a node and a string; the node is the
base expression, and the string is the attribute that was accessed::

    >>> class Builder(Builder):
    ...     def Getattr(self, expr, attr):
    ...         return 'Getattr(%s,%r)' % (build(self,expr), attr)

    >>> pe("a.b")
    Getattr(a,'b')

    >>> pe("a.b.c")
    Getattr(Getattr(a,'b'),'c')


Simple Binary Operators
=======================

There are 10 "simple" binary operator methods, that take a pair of left and
right nodelists as arguments::

    >>> class Builder(Builder):
    ...     def mkBinOp(op):
    ...         pat = op + '(%s,%s)'
    ...         def method(self, left, right):
    ...             return pat % (build(self,left), build(self,right))
    ...         return method
    ...
    ...     Add        = mkBinOp('Add')
    ...     Sub        = mkBinOp('Sub')
    ...     Mul        = mkBinOp('Mul')
    ...     Div        = mkBinOp('Div')
    ...     Mod        = mkBinOp('Mod')
    ...     FloorDiv   = mkBinOp('FloorDiv')
    ...     Power      = mkBinOp('Power')
    ...     LeftShift  = mkBinOp('LeftShift')
    ...     RightShift = mkBinOp('RightShift')
    ...     Subscript  = mkBinOp('Subscript')

Most of these operators correspond to normal Python binary operators::

    >>> pe("a+b")
    Add(a,b)
    >>> pe("b-a")
    Sub(b,a)
    >>> pe("c*d")
    Mul(c,d)
    >>> pe("c/d")
    Div(c,d)
    >>> pe("c%d")
    Mod(c,d)
    >>> pe("c//d")
    FloorDiv(c,d)
    >>> pe("a**b")
    Power(a,b)
    >>> pe("a<<b")
    LeftShift(a,b)
    >>> pe("a>>b")
    RightShift(a,b)

    >>> pe("a[1]")
    Subscript(a,1)
    >>> pe("a[1][2]")
    Subscript(Subscript(a,1),2)

By the way, ``Ellipsis`` is also handled by the ``Const`` method, in the case
where you have an expression like ``foo[...]``::

    >>> pe("a[...]")
    Subscript(a,Ellipsis)


"List" Operators
================

The 7 "list operator" methods take a single argument: a sequence of nodes
that represent a list of expressions delimited by the corresponding operator::

    >>> class Builder(Builder):
    ...     def multiOp(fmt,sep=','):
    ...         def method(self,items):
    ...             return fmt % sep.join([build(self,item) for item in items])
    ...         return method
    ...
    ...     And        = multiOp('And(%s)')
    ...     Or         = multiOp('Or(%s)')
    ...     Tuple      = multiOp('Tuple(%s)')
    ...     List       = multiOp('List(%s)')
    ...     Bitor      = multiOp('Bitor(%s)')
    ...     Bitxor     = multiOp('Bitxor(%s)')
    ...     Bitand     = multiOp('Bitand(%s)')

    >>> pe("a and b")
    And(a,b)
    >>> pe("a or b")
    Or(a,b)
    >>> pe("a and b and c")
    And(a,b,c)
    >>> pe("a or b or c")
    Or(a,b,c)
    >>> pe("a and b and c and d")
    And(a,b,c,d)
    >>> pe("a or b or c or d")
    Or(a,b,c,d)

    >>> pe("a&b&c")
    Bitand(a,b,c)
    >>> pe("a|b|c")
    Bitor(a,b,c)
    >>> pe("a^b^c")
    Bitxor(a,b,c)

    >>> pe("a&b&c&d")
    Bitand(a,b,c,d)
    >>> pe("a|b|c|d")
    Bitor(a,b,c,d)
    >>> pe("a^b^c^d")
    Bitxor(a,b,c,d)


Tuples
------

No parens::

    >>> pe("a,")
    Tuple(a)
    >>> pe("a,b")
    Tuple(a,b)
    >>> pe("a,b,c")
    Tuple(a,b,c)
    >>> pe("a,b,c,")
    Tuple(a,b,c)

With parens::

    >>> pe("()")
    Tuple()
    >>> pe("(a)")
    a
    >>> pe("(a,)")
    Tuple(a)
    >>> pe("(a,b)")
    Tuple(a,b)
    >>> pe("(a,b,)")
    Tuple(a,b)
    >>> pe("(a,b,c)")
    Tuple(a,b,c)
    >>> pe("(a,b,c,)")
    Tuple(a,b,c)


Lists
-----

::

    >>> pe("[]")
    List()
    >>> pe("[a]")
    List(a)
    >>> pe("[a,]")
    List(a)
    >>> pe("[a,b]")
    List(a,b)
    >>> pe("[a,b,]")
    List(a,b)
    >>> pe("[a,b,c]")
    List(a,b,c)
    >>> pe("[a,b,c,]")
    List(a,b,c)



Slicing
=======

The ``Slice2`` method takes two arguments: a start and stop value.  Each is
either a node list or ``None`` (in which case there is no expression for that
part of the slice)::

    >>> class Builder(Builder):
    ...     def Slice2(self,start,stop):
    ...         txt = 'Slice('
    ...         if start:
    ...             txt += build(self, start)
    ...         txt += ':'
    ...         if stop:
    ...             txt += build(self, stop)
    ...         return txt+')'

    >>> pe("a[:]")
    Subscript(a,Slice(:))

    >>> pe("a[1:2]")
    Subscript(a,Slice(1:2))

    >>> pe("a[1:]")
    Subscript(a,Slice(1:))

    >>> pe("a[:2]")
    Subscript(a,Slice(:2))


The ``Slice3`` method is similar, but takes three arguments::

    >>> class Builder(Builder):
    ...     def Slice3(self,start,stop,stride):
    ...         txt = 'Slice('
    ...         if start:
    ...             txt += build(self, start)
    ...         txt += ':'
    ...         if stop:
    ...             txt += build(self, stop)
    ...         txt += ':'
    ...         if stride:
    ...             txt += build(self, stride)
    ...         return txt+')'

    >>> pe("a[::]")
    Subscript(a,Slice(::))

    >>> pe("a[1::]")
    Subscript(a,Slice(1::))

    >>> pe("a[:2:]")
    Subscript(a,Slice(:2:))

    >>> pe("a[1:2:]")
    Subscript(a,Slice(1:2:))

    >>> pe("a[::3]")
    Subscript(a,Slice(::3))

    >>> pe("a[1::3]")
    Subscript(a,Slice(1::3))

    >>> pe("a[:2:3]")
    Subscript(a,Slice(:2:3))

    >>> pe("a[1:2:3]")
    Subscript(a,Slice(1:2:3))


Comparisons and Conditional Expressions
=======================================

The ``Compare`` method receives two arguments: a node for the first expression
to be compared, followed by a list of ``(op, expr)`` tuples for subsequent
comparisons.  The ``op`` value is a string representing the comparison operator
used, and each ``expr`` is a node::

    >>> class Builder(Builder):
    ...     def Compare(self,initExpr,comparisons):
    ...         data = [build(self,initExpr)]
    ...         for op,val in comparisons:
    ...             data.append(op)
    ...             data.append(build(self,val))
    ...         return 'Compare(%s)' % ' '.join(data)

    >>> pe("a>b")
    Compare(a > b)
    >>> pe("a>=b")
    Compare(a >= b)
    >>> pe("a<b")
    Compare(a < b)
    >>> pe("a<=b")
    Compare(a <= b)
    >>> pe("a<>b")
    Compare(a <> b)
    >>> pe("a!=b")
    Compare(a != b)
    >>> pe("a==b")
    Compare(a == b)
    >>> pe("a in b")
    Compare(a in b)
    >>> pe("a is b")
    Compare(a is b)
    >>> pe("a not in b")
    Compare(a not in b)
    >>> pe("a is not b")
    Compare(a is not b)

N-Way Comparisons
-----------------

If you don't want to have to process N-way comparisons in your builder, you
can set the ``simplify_comparisons`` flag on your class to ``True``, and N-way
comparisons will be converted to ``and`` expressions::

    >>> Builder.simplify_comparisons = True

    >>> pe("1<2<3")
    And(Compare(1 < 2),Compare(2 < 3))

Otherwise, you can set the flag to ``False``, and you will receive N-way
comparisons as additional values in the list of ``(op, expr)`` tuples::

    >>> Builder.simplify_comparisons = False

    >>> pe("1<2<3")
    Compare(1 < 2 < 3)

    >>> pe("a>=b>c<d")
    Compare(a >= b > c < d)

Note that you *must* explicitly set ``simplify_comparisons`` to either a true
or false value; there is no default.


Conditional Expressions
-----------------------

The ``IfElse`` method receives three arguments: a node for the "true" value,
a node for the condition, and a node for the "false" value::

    >>> class Builder(Builder):
    ...     def IfElse(self, trueVal, condition, falseVal):
    ...         return 'IfElse(%s, %s, %s)' % (
    ...             build(self, trueVal), build(self, condition),
    ...             build(self, falseVal)
    ...         )

    >>> import sys
    >>> if sys.version>='2.5':
    ...     pe("a if b else c")
    ... else:
    ...     print "IfElse(a, b, c)"
    IfElse(a, b, c)



List Comprehensions and Generator Expressions
=============================================

The ``ListComp`` and ``GenExpr`` methods receive two arguments: a node for the
output expression, and a list of ``(op, node)`` tuples, where `op` is the name
of an operator (either "for", "in", or "if"), and `node` is the node
corresponding to the operator's argument::

    >>> class Builder(Builder):
    ...     def ListComp(self, initExpr, clauses):
    ...         data = [build(self,initExpr)]
    ...         for op,val in clauses:
    ...             data.append(op)
    ...             data.append(build(self,val))
    ...         return 'ListComp(%s)' % ' '.join(data)
    ...
    ...     def GenExpr(self, initExpr, clauses):
    ...         data = [build(self,initExpr)]
    ...         for op,val in clauses:
    ...             data.append(op)
    ...             data.append(build(self,val))
    ...         return 'GenExpr(%s)' % ' '.join(data)

    >>> pe("[x for x in y if z]")
    ListComp(x for x in y if z)

    >>> pe("[(x+1, 42) for x in y for y in z if q>z]")
    ListComp(Tuple(Add(x,1),42) for x in y for y in z if Compare(q > z))

    >>> pe("[x+1 for x in y if x in z for y in q if r if p]")
    ListComp(Add(x,1) for x in y if Compare(x in z) for y in q if r if p)

    >>> if sys.version>='2.4':
    ...     pe("(x for x in y if z)")
    ... else:
    ...     print "GenExpr(x for x in y if z)"
    GenExpr(x for x in y if z)

Note, by the way, that when you are building the "for" clause assignments,
you'll need to handle arbitrary assignments (e.g. tuple unpacking)::

    >>> pe("[x for y, x in z]")
    ListComp(x for Tuple(y,x) in z)

    >>> pe("[x.y for x.y in z]")
    ListComp(Getattr(x,'y') for Getattr(x,'y') in z)

    >>> pe("[x[y] for x[y] in z]")
    ListComp(Subscript(x,y) for Subscript(x,y) in z)

(Normally, you would handle this by passing the "for" clauses to a different
builder instance that's set up to handle calls to ``Name``, ``Getattr``,
``Tuple``, etc. by generating assignments instead of lookups.)


Dictionaries
============

The ``Dict`` method takes one argument: a list of ``(key, value)`` tuples,
where both the keys and values are expression nodes::

    >>> class Builder(Builder):
    ...     def Dict(self, items):
    ...         return '{%s}' % ','.join([
    ...             '%s:%s' % (build(self,k),build(self,v)) for k,v in items
    ...         ])

    >>> pe("{ (a,b):c+d, e:[f,g]  }")
    {Tuple(a,b):Add(c,d),e:List(f,g)}


Calls
=====

The ``CallFunc`` method takes five arguments:

`func`
    The expression to be called.

`args`
    A list of positional argument expression nodes.

`kw`
    A list of ``(argname, value)`` expression node pairs

`star_node`
    The node for the ``*args`` expression, or ``None`` if there isn't one.

`dstar_node`
    The node for the ``*kw`` expression, or ``None`` if there isn't one.


    >>> class Builder(Builder):
    ...     def CallFunc(self, func, args, kw, star_node, dstar_node):
    ...         if star_node:
    ...             star_node=build(self, star_node)
    ...         else:
    ...             star_node = 'None'
    ...         if dstar_node:
    ...             dstar_node=build(self, dstar_node)
    ...         else:
    ...             dstar_node = 'None'
    ...         return 'Call(%s,%s,%s,%s,%s)' % (
    ...             build(self,func), self.Tuple(args), self.Dict(kw),
    ...             star_node, dstar_node
    ...         )

    >>> pe("a()")
    Call(a,Tuple(),{},None,None)

    >>> pe("a(1,2)")
    Call(a,Tuple(1,2),{},None,None)
    >>> pe("a(1,2,)")
    Call(a,Tuple(1,2),{},None,None)
    >>> pe("a(b=3)")
    Call(a,Tuple(),{'b':3},None,None)
    >>> pe("a(1,2,b=3)")
    Call(a,Tuple(1,2),{'b':3},None,None)

    >>> pe("a(*x)")
    Call(a,Tuple(),{},x,None)
    >>> pe("a(1,*x)")
    Call(a,Tuple(1),{},x,None)
    >>> pe("a(b=3,*x)")
    Call(a,Tuple(),{'b':3},x,None)
    >>> pe("a(1,2,b=3,*x)")
    Call(a,Tuple(1,2),{'b':3},x,None)

    >>> pe("a(**y)")
    Call(a,Tuple(),{},None,y)
    >>> pe("a(1,**y)")
    Call(a,Tuple(1),{},None,y)
    >>> pe("a(b=3,**y)")
    Call(a,Tuple(),{'b':3},None,y)
    >>> pe("a(1,2,b=3,**y)")
    Call(a,Tuple(1,2),{'b':3},None,y)

    >>> pe("a(*x,**y)")
    Call(a,Tuple(),{},x,y)
    >>> pe("a(1,*x,**y)")
    Call(a,Tuple(1),{},x,y)
    >>> pe("a(b=3,*x,**y)")
    Call(a,Tuple(),{'b':3},x,y)
    >>> pe("a(1,2,b=3,*x,**y)")
    Call(a,Tuple(1,2),{'b':3},x,y)


    >>> if sys.version>='2.4':
    ...     pe("a(x for x in y if z)")
    ...     pe("a(x for x in y if z, q)")
    ... else:
    ...     print "Call(a,Tuple(GenExpr(x for x in y if z)),{},None,None)"
    ...     print "Call(a,Tuple(GenExpr(x for x in y if z),q),{},None,None)"
    Call(a,Tuple(GenExpr(x for x in y if z)),{},None,None)
    Call(a,Tuple(GenExpr(x for x in y if z),q),{},None,None)


Miscellaneous Tests
===================

An interesting quirk of the AST module is that it supports parsing some calls
that *should* be syntax errors.  The ``ast_builder`` module thus has to trap
these itself::

    >>> pe("a(1=2)")    # expr as kw
    Traceback (most recent call last):
      ...
    SyntaxError: keyword can't be an expression (...)

    >>> pe("a(b=2,c)")
    Traceback (most recent call last):
      ...
    SyntaxError: non-keyword arg after keyword arg


Most of Python's operator associativity and precedence is grammar-driven, but
certain parts have to be handled by ``ast_builder``.  These are just some tests
to make sure that associativity is correct::

    >>> pe("a+b+c")
    Add(Add(a,b),c)
    >>> pe("a*b*c")
    Mul(Mul(a,b),c)
    >>> pe("a/b/c")
    Div(Div(a,b),c)
    >>> pe("a//b//c")
    FloorDiv(FloorDiv(a,b),c)
    >>> pe("a%b%c")
    Mod(Mod(a,b),c)
    >>> pe("a<<b<<c")
    LeftShift(LeftShift(a,b),c)
    >>> pe("a>>b>>c")
    RightShift(RightShift(a,b),c)
    >>> pe("a()()")
    Call(Call(a,Tuple(),{},None,None),Tuple(),{},None,None)

    >>> pe("a**b**c")   # power is right-associative
    Power(a,Power(b,c))

    >>> pe("5*x**2 + 4*x + -1")
    Add(Add(Mul(5,Power(x,2)),Mul(4,x)),Minus(1))