Dispelling py.test magic

How to replicate py.test magic in lest than 100 lines of code.
Demo code available here: https://github.com/oinopion/dispel

1. Dispelling py.test magic Tomek Paczkowski PyCon UK
 September 2015
2. py.test is awesome Test discovery No boilerplate Plugins Asserting using assert statement
3. def double(x): return x * 2 def test_doubling(): expected = 5 assert double(2) == expected
4. def test_doubling(): expected = 5 > assert double(2) == expected E assert 4 == 5 E + where 4 = double(2) sample_test.py:6: AssertionError
5. def test_doubling(): expected = 5 > assert double(2) == expected E assert 4 == 5 E + where 4 = double(2) sample_test.py:6: AssertionError
6. How to replicate this?
7. Benjamin Peterson Behind the scenes of py.test's new assertion rewriting bit.ly/pytest-ast
8. Step one import ast
9. assert double(2) == expected assert == func call variable constant variable
10. ast.parse('assert double(2) == expected') bit.ly/docs-ast
11. Assert( test=Compare( left=Call( func=Name(id='double'), args=[Num(n=2)] ), ops=[Eq()], comparators=[Name(id='expected')] ) )
12. Assert( test=Compare( left=Call( func=Name(id='double'), args=[Num(n=2)] ), ops=[Eq()], comparators=[Name(id='expected')] ) )
13. Assert( test=Compare( left=Call( func=Name(id='double'), args=[Num(n=2)] ), ops=[Eq()], comparators=[Name(id='expected')] ) )
14. Assert( test=Compare( left=Call( func=Name(id='double'), args=[Num(n=2)] ), ops=[Eq()], comparators=[Name(id='expected')] ) )
15. Assert( test=Compare( left=Call( func=Name(id='double'), args=[Num(n=2)] ), ops=[Eq()], comparators=[Name(id='expected')] ) )
16. Assert( test=Compare( left=Call( func=Name(id='double'), args=[Num(n=2)] ), ops=[Eq()], comparators=[Name(id='expected')] ) )
17. Assert( test=Compare( left=Call( func=Name(id='double'), args=[Num(n=2)] ), ops=[Eq()], comparators=[Name(id='expected')] ) ) bit.ly/better-docs-ast
18. Goal one Modify the assert statement to
 call a function with both sides 
 of the comparison
19. assert double(2) == expected assert_equals(double(2), expected)
20. Node transformer Allows AST tree modification Implements visitor pattern
21. class AssertRewrite(NodeTransformer):
 def visit_Assert(self, node):
 call = Call(
 func=Name(
 id='assert_equals', ctx=Load()
 ),
 args=[
 node.test.left,
 node.test.comparators[0]
 ],
 keywords=[]
 )
 new_node = Expr(value=call)
 copy_location(new_node, node)
 fix_missing_locations(new_node)
 return new_node
22. class AssertRewrite(NodeTransformer):
 def visit_Assert(self, node):
 call = Call(
 func=Name(
 id='assert_equals', ctx=Load()
 ),
 args=[
 node.test.left,
 node.test.comparators[0]
 ],
 keywords=[]
 )
 new_node = Expr(value=call)
 copy_location(new_node, node)
 fix_missing_locations(new_node)
 return new_node
23. class AssertRewrite(NodeTransformer):
 def visit_Assert(self, node):
 call = Call(
 func=Name(
 id='assert_equals', ctx=Load()
 ),
 args=[
 node.test.left,
 node.test.comparators[0]
 ],
 keywords=[]
 )
 new_node = Expr(value=call)
 copy_location(new_node, node)
 fix_missing_locations(new_node)
 return new_node
24. class AssertRewrite(NodeTransformer):
 def visit_Assert(self, node):
 call = Call(
 func=Name(
 id='assert_equals', ctx=Load()
 ),
 args=[
 node.test.left,
 node.test.comparators[0]
 ],
 keywords=[]
 )
 new_node = Expr(value=call)
 copy_location(new_node, node)
 fix_missing_locations(new_node)
 return new_node
25. class AssertRewrite(NodeTransformer):
 def visit_Assert(self, node):
 call = Call(
 func=Name(
 id='assert_equals', ctx=Load()
 ),
 args=[
 node.test.left,
 node.test.comparators[0]
 ],
 keywords=[]
 )
 new_node = Expr(value=call)
 copy_location(new_node, node)
 fix_missing_locations(new_node)
 return new_node
26. class AssertRewrite(NodeTransformer):
 def visit_Assert(self, node):
 call = Call(
 func=Name(
 id='assert_equals', ctx=Load()
 ),
 args=[
 node.test.left,
 node.test.comparators[0]
 ],
 keywords=[]
 )
 new_node = Expr(value=call)
 copy_location(new_node, node)
 fix_missing_locations(new_node)
 return new_node
27. class AssertRewrite(NodeTransformer):
 def visit_Assert(self, node):
 call = Call(
 func=Name(
 id='assert_equals', ctx=Load()
 ),
 args=[
 node.test.left,
 node.test.comparators[0]
 ],
 keywords=[]
 )
 new_node = Expr(value=call)
 copy_location(new_node, node)
 fix_missing_locations(new_node)
 return new_node
28. class AssertRewrite(NodeTransformer):
 def visit_Assert(self, node):
 call = Call(
 func=Name(
 id='assert_equals', ctx=Load()
 ),
 args=[
 node.test.left,
 node.test.comparators[0]
 ],
 keywords=[]
 )
 new_node = Expr(value=call)
 copy_location(new_node, node)
 fix_missing_locations(new_node)
 return new_node
29. Where is assert_equals coming from?
30. def transform(module):
 import_node = ImportFrom(
 module='test_utils',
 names=[alias('assert_equals', None)],
 lineno=0, col_offset=0,
 )
 module.body[0:0] = [import_node]
 transformer = AssertRewrite()
 return transformer.visit(module)
31. def transform(module):
 import_node = ImportFrom(
 module='test_utils',
 names=[alias('assert_equals', None)],
 lineno=0, col_offset=0,
 )
 module.body[0:0] = [import_node]
 transformer = AssertRewrite()
 return transformer.visit(module)
32. def transform(module):
 import_node = ImportFrom(
 module='test_utils',
 names=[alias('assert_equals', None)],
 lineno=0, col_offset=0,
 )
 module.body[0:0] = [import_node]
 transformer = AssertRewrite()
 return transformer.visit(module)
33. def transform(module):
 import_node = ImportFrom(
 module='test_utils',
 names=[alias('assert_equals', None)],
 lineno=0, col_offset=0,
 )
 module.body[0:0] = [import_node]
 transformer = AssertRewrite()
 return transformer.visit(module)
34. def transform(module):
 import_node = ImportFrom(
 module='test_utils',
 names=[alias('assert_equals', None)],
 lineno=0, col_offset=0,
 )
 module.body[0:0] = [import_node]
 transformer = AssertRewrite()
 return transformer.visit(module)
35. def transform(module):
 import_node = ImportFrom(
 module='test_utils',
 names=[alias('assert_equals', None)],
 lineno=0, col_offset=0,
 )
 module.body[0:0] = [import_node]
 transformer = AssertRewrite()
 return transformer.visit(module)
36. import_node = ImportFrom(
 module='test_utils',
 names=[alias('assert_equals', None)],
 lineno=0, col_offset=0,
 )
37. import_node = ImportFrom(
 module='test_utils',
 names=[alias('assert_equals', '#eq')],
 lineno=0, col_offset=0,
 ) Call(
 func=Name(id='assert_equals', ctx=Load()),
 args=[...],
 keywords=[]
 )
38. import_node = ImportFrom(
 module='test_utils',
 names=[alias('assert_equals', '#eq')],
 lineno=0, col_offset=0,
 ) Call(
 func=Name(id='#eq', ctx=Load()),
 args=[...],
 keywords=[]
 )
39. Step two import sys
40. Import path hooks sys.path_hooks Factory functions for finders sys.path_importer_cache bit.ly/docs-import
41. Goal two Write an import hook 
 that uses our transformer 
 to modify imported code
42. def import_hook(path): if os.path.abspath('') == path: return Finder() else: raise ImportError sys.path_hooks.insert(0, import_hook) sys.path_importer_cache.clear()
43. def import_hook(path): if os.path.abspath('') == path: return Finder() else: raise ImportError sys.path_hooks.insert(0, import_hook) sys.path_importer_cache.clear()
44. def import_hook(path): if os.path.abspath('') == path: return Finder() else: raise ImportError sys.path_hooks.insert(0, import_hook) sys.path_importer_cache.clear()
45. def import_hook(path): if os.path.abspath('') == path: return Finder() else: raise ImportError sys.path_hooks.insert(0, import_hook) sys.path_importer_cache.clear()
46. def import_hook(path): if os.path.abspath('') == path: return Finder() else: raise ImportError sys.path_hooks.insert(0, import_hook) sys.path_importer_cache.clear()
47. Finder Defines one method: find_spec Method returns a ModuleSpec
48. from importlib.util import spec_from_file_location
 
 class Finder:
 def find_spec(self, module, target=None):
 file_name = module + '.py'
 if not os.path.exists(file_name):
 return None
 return spec_from_file_location(
 name=module, 
 location=file_name, 
 loader=Loader()
 )
49. from importlib.util import spec_from_file_location
 
 class Finder:
 def find_spec(self, module, target=None):
 file_name = module + '.py'
 if not os.path.exists(file_name):
 return None
 return spec_from_file_location(
 name=module, 
 location=file_name, 
 loader=Loader()
 )
50. from importlib.util import spec_from_file_location
 
 class Finder:
 def find_spec(self, module, target=None):
 file_name = module + '.py'
 if not os.path.exists(file_name):
 return None
 return spec_from_file_location(
 name=module, 
 location=file_name, 
 loader=Loader()
 )
51. from importlib.util import spec_from_file_location
 
 class Finder:
 def find_spec(self, module, target=None):
 file_name = module + '.py'
 if not os.path.exists(file_name):
 return None
 return spec_from_file_location(
 name=module, 
 location=file_name, 
 loader=Loader()
 )
52. from importlib.util import spec_from_file_location
 
 class Finder:
 def find_spec(self, module, target=None):
 file_name = module + '.py'
 if not os.path.exists(file_name):
 return None
 return spec_from_file_location(
 name=module, 
 location=file_name, 
 loader=Loader()
 )
53. from importlib.util import spec_from_file_location
 
 class Finder:
 def find_spec(self, module, target=None):
 file_name = module + '.py'
 if not os.path.exists(file_name):
 return None
 return spec_from_file_location(
 name=module, 
 location=file_name, 
 loader=Loader()
 )
54. Loader Defines one method: exec_module Executes module code Populates module namespace
55. class Loader: def exec_module(self, module): with open(module.__file__, 'rb') as fp: source = fp.read() tree = ast.parse(source, module.__file__) tree = transform(tree) code = compile(tree, module.__file__, 'exec') exec(code, module.__dict__)
56. class Loader: def exec_module(self, module): with open(module.__file__, 'rb') as fp: source = fp.read() tree = ast.parse(source, module.__file__) tree = transform(tree) code = compile(tree, module.__file__, 'exec') exec(code, module.__dict__)
57. class Loader: def exec_module(self, module): with open(module.__file__, 'rb') as f: source = f.read() tree = ast.parse(source, module.__file__) tree = transform(tree) code = compile(tree, module.__file__, 'exec') exec(code, module.__dict__)
58. class Loader: def exec_module(self, module): with open(module.__file__, 'rb') as fp: source = fp.read() tree = ast.parse(source, module.__file__) tree = transform(tree) code = compile(tree, module.__file__, 'exec') exec(code, module.__dict__)
59. class Loader: def exec_module(self, module): with open(module.__file__, 'rb') as fp: source = fp.read() tree = ast.parse(source, module.__file__) tree = transform(tree) code = compile(tree, module.__file__, 'exec') exec(code, module.__dict__)
60. class Loader: def exec_module(self, module): with open(module.__file__, 'rb') as fp: source = fp.read() tree = ast.parse(source, module.__file__) tree = transform(tree) code = compile(tree, module.__file__, 'exec') exec(code, module.__dict__)
61. class Loader: def exec_module(self, module): with open(module.__file__, 'rb') as fp: source = fp.read() tree = ast.parse(source, module.__file__) tree = transform(tree) code = compile(tree, module.__file__, 'exec') exec(code, module.__dict__)
62. class Loader: def exec_module(self, module): with open(module.__file__, 'rb') as fp: source = fp.read() tree = ast.parse(source, module.__file__) tree = transform(tree) code = compile(tree, module.__file__, 'exec') exec(code, module.__dict__)
63. class Loader:
 def exec_module(self, module):
 with open(module.__file__, 'rb') as fp:
 source = fp.read()
 
 tree = ast.parse(source, module.__file__)
 tree = transform(tree)
 code = compile(tree, module.__file__, 'exec')
 module.__dict__['#eq'] = assert_equals
 exec(code, module.__dict__) Bonus
64. Step three Test discovery
65. import sample_test sample_test.test_with_assert()
66. Demo
67. Summary This is probably a giant foot gun Corner cases left for the reader Python is awesome
68. This presentation
 bit.ly/dispel-pytest Demo code
 github.com/oinopion/dispel
69. Thanks! Tomek Paczkowski @oinopion