A desugaring phase for a Python compiler project
Racket Makefile
Switch branches/tags
Nothing to show
Clone or download
Fetching latest commit…
Cannot retrieve the latest commit at this time.
Failed to load latest commit information.


Project: Desugar Python (Phase I)

In this project, you will desugar constructs in Python.

Assume Python has been parsed into the format described in the python-to-s-expression converter for the input.

Provided utilities

Two utilities have been provided to make writing this phase easier: the pywalk package and sxpy.


The pywalk package provides facilities for walking over Python abstract syntax trees and making point transformations.


The sxpy utility converts a Python AST in s-expression form back into running Python code.

Required transformations

Canonicalize return

Return should always take a return value.

In the case of a blank return:


it should become:

  return None

Decorator dropping

Decorators should be dropped to function calls:

def <name>(<parameters>):


def <name>(<parameters>):
<name> = <decorator>(<name>)

just as:

class <name>(<parameters>): 


class <name>(<parameters>): 
<name> = <decorator>(<name>)

Default dropping

Default values for parameters should be dropped into assignments after the procedure definition.

For example:

def f(x = g()): pass


def f(x): pass
f.__defaults__  = (g(),)
f.__kwdefaults__ = None


def f(a=10,*,b=20): pass


def f(a,*,b): pass

f.__defaults__ = (10,)
f.__kwdefaults__ = {"b":20}

Annotation dropping

Annotations (types) on parameters should be dropped to an assignment below a procedure.

For example,

def f(x : int) -> int:
  return x + 1

should become:

def f(x):
  return (x) + (1)

f.__annotations__ = {"return":int,"x":int}

for-loop elimination

All for loop should be converted to while loops.

The following transformation is recommended:

for <targets> in <iterator>:


$tmp2 = ($tmp1 for $tmp1 in <iterator>)
while True:
    <targets> = $tmp2.__next__()
  except StopIteration:

Assignment flattening / *-elimination

Starred expressions, lists and tuples should be eliminated from assignment forms by flattening assignments out.

For instance:

(a,b,c) = somelist


$tmp = list(somelist)
a = ($tmp)[0]
b = ($tmp)[1]
c = ($tmp)[2]


(a,b,*c,d,e) = elts


$tmp = list(elts)
a = ($tmp)[0]
b = ($tmp)[1]
c = ($tmp)[2:-2:]
d = ($tmp)[-2]
e = ($tmp)[-1]

Class elimination

Because classes in Python are objects and the type primitive can create classes dynamically, all class declarations can be eliminated and desugared into a combination of function definitions and function calls.

For instance:

class X:

  y = 10

x = X()



def $tmp1($tmp2 = object,*$tmp3,metaclass = type,**$tmp4):
  __dict__ = {}
  (__dict__)["y"] = 10
  return metaclass("X",($tmp2,) + $tmp3,__dict__)

X = $tmp1()
x = X()


  1. The transformation need not be "error-raise-preserving."

    That is, you may opt to use a transformation that does not raise an Error-level exception when Python would have raised an Error-level exception.

    Rather than raise an error, your implementation may do anything.

  2. You may assume that the names in builtins are never assigned. (This will make it easier to write hygienic transformations that want to desugar into builtins.) For example, you can assume list will always be the list class.

Output grammar

The output must conform to the following grammar:

<mod> ::= (Module <stmt>*)

<stmt> ::=

          (name <identifier>)
          ; NOTE: <arguments> is simplified below.
          (args <arguments>) 
          (body <stmt>*)
          (returns #f))

      | (Return <expr>) 

      | (Delete <expr>*) 

      ; NOTE: <expr> no longer has starred form.
      | (Assign (targets <expr>) (value <expr>)) 

      | (AugAssign <expr> <operator> <expr>)  

      | (While (test <expr>) (body <stmt>*) (orelse <stmt>*))

      | (If (test <expr>) (body <stmt>*) (orelse <stmt>*))

      | (With [<withitem>*] <stmt>*)

      | (Raise <expr>)  
      | (Raise <expr> <expr>)

      | (Try (body <stmt>*)
             (handlers <excepthandler>*)
             (orelse <stmt>*)
             (finalbody <stmt>*))

      | (Assert <expr>)  
      | (Assert <expr> <expr>)

      | (Import <alias>*)
      | (ImportFrom (module <identifier?>)
                    (names <alias>*)
                    (level <int?>))

      | (Global <identifier>+)
      | (Nonlocal <identifier>+)

      | (Expr <expr>)

      | (Pass) 
      | (Break)
      | (Continue)

      ;; Added:

      ; Use Local to specify variables assinged here:
      | (Local <identifier>*) 

      ; Use Comment to specify a comment (useful in debugging):
      | (Comment <string>)

<expr> ::=
       (BoolOp <boolop> <expr>*)
     | (BinOp <expr> <operator> <expr>)
     | (UnaryOp <unaryop> <expr>)

     | (Lambda <arguments> <expr>)

     | (IfExp <expr> <expr> <expr>)

     | (Dict (keys <expr>*) (values <expr>*))
     | (Set <expr>*)
     | (ListComp <expr> <comprehension>*)
     | (SetComp <expr> <comprehension>*) ; call to ListComp
     | (DictComp <expr> <expr> <comprehension>*)

     | (GeneratorExp <expr> <comprehension>*)

     | (Yield)  | (Yield <expr>)

     | (YieldFrom <expr>)

     | (Compare (left        <expr>) 
                (ops         <cmpop>*)
                (comparators <expr>*))

     | (Call (func <expr>)
             (args <expr>*)
             (keywords <keyword>*)
             (starags <expr?>)
             (kwargs <expr?>))

     | (Num <number>)
     | (Str <string>)
     | (Bytes <byte-string>)

     | (NameConstant <name-constant>)
     | (Ellipsis)

     ; the following expression can appear in assignment context:
     ; NOTE: Starred form has been eliminated:
     | (Attribute <expr> <identifier>)
     | (Subscript <expr> <slice>)
     | (Name <identifier>)
     | (List <expr>*)
     | (Tuple <expr>*)

<name-constant> ::= True | False | None

<slice> ::= (Slice <expr?> <expr?> <expr?>)
         |  (ExtSlice <slice>*) 
         |  (Index <expr>)

<boolop> ::= And | Or 

<operator> ::= Add | Sub | Mult | Div | Mod | Pow | LShift 
               | RShift | BitOr | BitXor | BitAnd | FloorDiv

<unaryop> ::= Invert | Not | UAdd | USub

<cmpop> ::= Eq | NotEq | Lt | LtE | Gt | GtE | Is | IsNot | In | NotIn

<comprehension> ::= [for <expr> in <expr> if <expr>*]

<excepthandler> ::= [except <expr?> <identifier?> <stmt>*]

<arguments> ::= (Arguments
                   (args <arg>*)
                   (arg-types #f*)
                   (vararg <arg?>) 
                   (kwonlyargs <arg>*)
                   (kwonlyarg-types #f*)
                   (kw_defaults #f*)
                   (kwarg <arg?>) 
                   (defaults #f*))
<arg> ::= <identifier>

<keyword> ::== [<identifier> <expr>]

<alias> ::= [<identifier> <identifier?>]

<withitem> ::= [<expr> <expr?>]

<arg?> ::= <arg> | #f

<expr?> ::= <expr> | #f

<int?> ::= <int> | #f

<identifier?> ::= <identifier> | #f