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

Transcript

1. Dispelling py.test
   magic
   Tomek Paczkowski
   PyCon UK

   September 2015
   

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2. py.test is awesome
   Test discovery
   No boilerplate
   Plugins
   Asserting using assert statement
   

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3. def double(x):
   return x * 2
   def test_doubling():
   expected = 5
   assert double(2) == expected
   

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4. def test_doubling():
   expected = 5
   > assert double(2) == expected
   E assert 4 == 5
   E + where 4 = double(2)
   sample_test.py:6: AssertionError
   

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5. def test_doubling():
   expected = 5
   > assert double(2) == expected
   E assert 4 == 5
   E + where 4 = double(2)
   sample_test.py:6: AssertionError
   

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6. How to replicate this?
   

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7. Benjamin Peterson
   Behind the scenes of py.test's
   new assertion rewriting
   bit.ly/pytest-ast
   

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8. Step one
   import ast
   

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9. assert double(2) == expected
   assert
   ==
   func call
   variable constant
   variable
   

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10. ast.parse('assert double(2) == expected')
    bit.ly/docs-ast
    

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11. Assert(
    test=Compare(
    left=Call(
    func=Name(id='double'),
    args=[Num(n=2)]
    ),
    ops=[Eq()],
    comparators=[Name(id='expected')]
    )
    )
    

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12. Assert(
    test=Compare(
    left=Call(
    func=Name(id='double'),
    args=[Num(n=2)]
    ),
    ops=[Eq()],
    comparators=[Name(id='expected')]
    )
    )
    

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13. Assert(
    test=Compare(
    left=Call(
    func=Name(id='double'),
    args=[Num(n=2)]
    ),
    ops=[Eq()],
    comparators=[Name(id='expected')]
    )
    )
    

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14. Assert(
    test=Compare(
    left=Call(
    func=Name(id='double'),
    args=[Num(n=2)]
    ),
    ops=[Eq()],
    comparators=[Name(id='expected')]
    )
    )
    

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15. Assert(
    test=Compare(
    left=Call(
    func=Name(id='double'),
    args=[Num(n=2)]
    ),
    ops=[Eq()],
    comparators=[Name(id='expected')]
    )
    )
    

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16. Assert(
    test=Compare(
    left=Call(
    func=Name(id='double'),
    args=[Num(n=2)]
    ),
    ops=[Eq()],
    comparators=[Name(id='expected')]
    )
    )
    

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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
    

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18. Goal one
    Modify the assert statement to

    call a function with both sides 

    of the comparison
    

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19. assert double(2) == expected
    assert_equals(double(2), expected)
    

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20. Node transformer
    Allows AST tree modification
    Implements visitor pattern
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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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
    

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29. Where is assert_equals
    coming from?
    

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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)
    

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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)
    

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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)
    

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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)
    

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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)
    

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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)
    

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36. import_node = ImportFrom(

    module='test_utils',

    names=[alias('assert_equals', None)],

    lineno=0, col_offset=0,

    )
    

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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=[]

    )
    

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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=[]

    )
    

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39. Step two
    import sys
    

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40. Import path hooks
    sys.path_hooks
    Factory functions for finders
    sys.path_importer_cache
    bit.ly/docs-import
    

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41. Goal two
    Write an import hook 

    that uses our transformer 

    to modify imported code
    

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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()
    

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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()
    

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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()
    

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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()
    

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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()
    

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47. Finder
    Defines one method: find_spec
    Method returns a ModuleSpec
    

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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()

    )
    

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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()

    )
    

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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()

    )
    

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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()

    )
    

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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()

    )
    

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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()

    )
    

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54. Loader
    Defines one method: exec_module
    Executes module code
    Populates module namespace
    

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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__)
    

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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__)
    

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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__)
    

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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__)
    

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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__)
    

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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__)
    

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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__)
    

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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__)
    

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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
    

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64. Step three
    Test discovery
    

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65. import sample_test
    sample_test.test_with_assert()
    

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66. Demo
    

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67. Summary
    This is probably a giant foot gun
    Corner cases left for the reader
    Python is awesome
    

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68. This presentation

    bit.ly/dispel-pytest
    Demo code

    github.com/oinopion/dispel
    

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69. Thanks!
    Tomek Paczkowski
    @oinopion
    

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