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"""
Python 3 to Python 2 converter (tree templates)
===============================================
This example demonstrates how to translate between two trees using tree templates.
It parses Python 3, translates it to a Python 2 AST, and then outputs the result as Python 2 code.
Uses reconstruct_python.py for generating the final Python 2 code.
"""
from lark import Lark
from lark.tree_templates import TemplateConf, TemplateTranslator
from lark.indenter import PythonIndenter
from reconstruct_python import PythonReconstructor
#
# 1. Define a Python parser that also accepts template vars in the code (in the form of $var)
#
TEMPLATED_PYTHON = r"""
%import python (single_input, file_input, eval_input, atom, var, stmt, expr, testlist_star_expr, _NEWLINE, _INDENT, _DEDENT, COMMENT, NAME)
%extend atom: TEMPLATE_NAME -> var
TEMPLATE_NAME: "$" NAME
?template_start: (stmt | testlist_star_expr _NEWLINE)
%ignore /[\t \f]+/ // WS
%ignore /\\[\t \f]*\r?\n/ // LINE_CONT
%ignore COMMENT
"""
parser = Lark(TEMPLATED_PYTHON, parser='lalr', start=['single_input', 'file_input', 'eval_input', 'template_start'], postlex=PythonIndenter(), maybe_placeholders=False)
def parse_template(s):
return parser.parse(s + '\n', start='template_start')
def parse_code(s):
return parser.parse(s + '\n', start='file_input')
#
# 2. Define translations using templates (each template code is parsed to a template tree)
#
pytemplate = TemplateConf(parse=parse_template)
translations_3to2 = {
'yield from $a':
'for _tmp in $a: yield _tmp',
'raise $e from $x':
'raise $e',
'$a / $b':
'float($a) / $b',
}
translations_3to2 = {pytemplate(k): pytemplate(v) for k, v in translations_3to2.items()}
#
# 3. Translate and reconstruct Python 3 code into valid Python 2 code
#
python_reconstruct = PythonReconstructor(parser)
def translate_py3to2(code):
tree = parse_code(code)
tree = TemplateTranslator(translations_3to2).translate(tree)
return python_reconstruct.reconstruct(tree)
#
# Test Code
#
_TEST_CODE = '''
if a / 2 > 1:
yield from [1,2,3]
else:
raise ValueError(a) from e
'''
def test():
print(_TEST_CODE)
print(' -----> ')
print(translate_py3to2(_TEST_CODE))
if __name__ == '__main__':
test()
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