Выступление на PYCON RUSSIA 2017

1. Debugging in Python 3.6:
Better, Faster, Stronger
Elizaveta Shashkova
JetBrains

2. Bio
• Software developer of PyCharm IDE at JetBrains
• Debugger
• Saint Petersburg
2

3. Debugging
• Adding print statements
• Logging
3

4. Debugging
• Adding print statements
• Logging
• Special tools: debuggers
4

5. Debugger’s Performance
5
Run
Debug
Debuggers are 30 times slower

6. Contents
• Tracing debugger
• Python 3.6
• Frame evaluation debugger
• Results
6

7. Contents
• Tracing debugger
• Python 3.6
• Frame evaluation debugger
• Results
7

8. 8
def tracefunc(frame, event, arg):
print(frame.f_lineno, event)
return tracefunc
sys.settrace(tracefunc)
1
2
3
4
5
6
sys.settrace(tracefunc) - set the
system tracing function
Tracing Function

9. def foo():
friends = ["Bob", "Tom"]
for f in friends:
print("Hi %s!” % f)
return len(friends)
sys.settrace(tracefunc)
foo()
1
2
3
4
5
6
7
8
9
Tracing Function
9

10. 1
2
3
4
5
6
7
8
9
1 calldef foo():
friends = ["Bob", "Tom"]
for f in friends:
print("Hi %s!” % f)
return len(friends)
sys.settrace(tracefunc)
foo()
Tracing Function
10

11. 1
2
3
4
5
6
7
8
9
1 call
2 line
def foo():
friends = ["Bob", "Tom"]
for f in friends:
print("Hi %s!” % f)
return len(friends)
sys.settrace(tracefunc)
foo()
Tracing Function
11

12. 1
2
3
4
5
6
7
8
9
1 call
2 line
3 line
4 line
Hi Bob!
def foo():
friends = ["Bob", "Tom"]
for f in friends:
print("Hi %s!” % f)
return len(friends)
sys.settrace(tracefunc)
foo()
Tracing Function
12

13. 1
2
3
4
5
6
7
8
9
1 call
2 line
3 line
4 line
Hi Bob!
3 line
4 line
Hi Tom!
def foo():
friends = ["Bob", "Tom"]
for f in friends:
print("Hi %s!” % f)
return len(friends)
sys.settrace(tracefunc)
foo()
Tracing Function
13

14. 1
2
3
4
5
6
7
8
9
1 call
2 line
3 line
4 line
Hi Bob!
3 line
4 line
Hi Tom!
5 line
5 return
def foo():
friends = ["Bob", "Tom"]
for f in friends:
print("Hi %s!” % f)
return len(friends)
sys.settrace(tracefunc)
foo()
Tracing Function
14

15. Build Python Debugger
• Breakpoints
• Stepping
15

16. Tracing Debugger
• Suspend program if breakpoint’s
line equals frame.f_lineno
• Handle events for stepping
16

17. Performance
def foo():
friends = ["Bob", "Tom"]
for f in friends:
print("Hi %s!” % f)
return len(friends)
sys.settrace(tracefunc)
foo()
1
2
3
4
5
6
7
8
9
1 call
2 line
3 line
4 line
Hi Bob!
3 line
4 line
Hi Tom!
5 line
5 return
17

18. Example 1
def calculate():
sum = 0
for i in range(10 ** 7):
sum += i
return sum
1
2
3
4
5
6
7
8
9
18

19. Example 1
def calculate():
sum = 0
for i in range(10 ** 7):
sum += i
return sum
def tracefunc(frame, event, arg):
return tracefunc
1
2
3
4
5
6
7
8
9
19

20. Performance
20
Run
Tracing
Breakpoints
0,80 sec
6,85 sec
19,81 sec

21. Performance
21
Run
Tracing
Breakpoints
0,80 sec
6,85 sec
19,81 sec

22. Performance
22
Run
Tracing
Breakpoints
0,80 sec
6,85 sec
19,81 sec

23. Performance
23
Run
Tracing
Breakpoints
0,80 sec
6,85 sec
19,81 sec
~ 25 times slower!

24. Problem
• Tracing call on every line
24

25. Optimizations
25

26. Optimization 1
• Skip scopes without breakpoints
• frame.f_code.co_name - name
of the function
26

27. Optimization 2
• Rewrite tracing function in C
27

28. Optimization 2
• Rewrite tracing function in C
• Works only for CPython
• ~ 4 times faster
28

29. Problem
• Tracing call on every line
29

30. Contents
• Tracing debugger
• Python 3.6
• Frame evaluation debugger
• Results
30

31. Contents
• Tracing debugger
• Python 3.6
• Frame evaluation debugger
• Results
31

32. Python 3.6
32

33. Python 3.6
• New frame evaluation API
• PEP 523
33

34. PEP 523
• Handle evaluation of frames
• Add a new field to code objects
• C API
34

35. Frame Evaluation
cdef frame_eval(PyFrameObject *frame, int exc):
func_name = <object> frame.f_code.co_name
line_number = <object> frame.f_lineno
print(line_number, func_name)
return _PyEval_EvalFrameDefault(frame, exc)
1
2
3
4
5
6
7
8
9
35

36. cdef frame_eval(PyFrameObject *frame, int exc):
func_name = <object> frame.f_code.co_name f
line_number = <object> frame.f_lineno
print(line_number, func_name)
return _PyEval_EvalFrameDefault(frame, exc)
1
2
3
4
5
6
7
8
9
Frame Evaluation
36

37. cdef frame_eval(PyFrameObject *frame, int exc):
func_name = <object> frame.f_code.co_name
line_number = <object> frame.f_lineno
print(line_number, func_name)
return _PyEval_EvalFrameDefault(frame, exc)
1
2
3
4
5
6
7
8
9
Frame Evaluation
37

38. cdef frame_eval(PyFrameObject *frame, int exc):
func_name = <object> frame.f_code.co_name
line_number = <object> frame.f_lineno
print(line_number, func_name)
return _PyEval_EvalFrameDefault(frame, exc)
1
2
3
4
5
6
7
8
9
Frame Evaluation
38

39. cdef frame_eval(PyFrameObject *frame, int exc):
func_name = <object> frame.f_code.co_name
line_number = <object> frame.f_lineno
print(line_number, func_name)
return _PyEval_EvalFrameDefault(frame, exc)
1
2
3
4
5
6
7
8
9
Frame Evaluation
39

40. cdef frame_eval(PyFrameObject *frame, int exc):
func_name = <object> frame.f_code.co_name
line_number = <object> frame.f_lineno
print(line_number, func_name)
return _PyEval_EvalFrameDefault(frame, exc)
def set_frame_eval():
cdef PyThreadState *state =
PyThreadState_Get()
state.interp.eval_frame = frame_eval
1
2
3
4
5
6
7
8
9
Frame Evaluation
40

41. Example
1
2
3
4
5
6
7
8
9
10
11
def first():
second()
def second():
third()
def third():
pass
set_frame_eval()
first()
41

42. Example
def first():
second()
def second():
third()
def third():
pass
set_frame_eval()
first()
1
2
3
4
5
6
7
8
9
10
11
1 first
4 second
7 third
42

43. Custom Frame Evaluation
• It works
• Executed while entering a frame
• Access to frame and code object
43

44. Contents
• Tracing debugger
• Python 3.6
• Frame evaluation debugger
• Results
44

45. Problem
• Tracing call on every line
45

46. Problem
• Tracing call on every line
• Remove the tracing function
46

47. Replace tracing
function with frame
evaluation function
47

48. Contents
• Tracing debugger
• Python 3.6
• Frame evaluation debugger
• Results
48

49. Build Python Debugger
• Breakpoints
• Stepping
49

50. Breakpoints
• Access to the whole code object
50

51. Breakpoints
• Access to the whole code object
• Insert breakpoint’s code into
frame’s code
51

52. Breakpoints
def maximum(a, b):
if a > b:
return a
else:
return b
1
2
3
4
5
6
7
8
52

53. Breakpoints
def maximum(a, b):
if a > b:
return a # breakpoint
else:
return b
1
2
3
4
5
6
7
8
53

54. Breakpoints
def maximum(a, b):
if a > b:
return a # breakpoint
else:
return b
1
2
3
4
5
6
7
8
breakpoint()
54

55. Breakpoints
def maximum(a, b):
if a > b:
breakpoint()
return a # breakpoint
else:
return b
1
2
3
4
5
6
7
8
55

56. Python Bytecode
def maximum(a, b):
if a > b:
return a
else:
return b
import dis
dis.dis(maximum)
1
2
3
4
5
6
7
8
56

57. Python Bytecode
def maximum(a, b):
if a > b:
return a
else:
return b
import dis
dis.dis(maximum)
1
2
3
4
5
6
7
8
2 0 LOAD_FAST 0 (a)
2 LOAD_FAST 1 (b)
4 COMPARE_OP 4 (>)
6 POP_JUMP_IF_FALSE 12
3 8 LOAD_FAST 0 (a)
10 RETURN_VALUE
5 >> 12 LOAD_FAST 1 (b)
14 RETURN_VALUE
16 LOAD_CONST 0 (None)
18 RETURN_VALUE
57

58. Python Bytecode
def maximum(a, b):
if a > b:
return a
else:
return b
import dis
dis.dis(maximum)
1
2
3
4
5
6
7
8
2 0 LOAD_FAST 0 (a)
2 LOAD_FAST 1 (b)
4 COMPARE_OP 4 (>)
6 POP_JUMP_IF_FALSE 12
3 8 LOAD_FAST 0 (a)
10 RETURN_VALUE
5 >> 12 LOAD_FAST 1 (b)
14 RETURN_VALUE
16 LOAD_CONST 0 (None)
18 RETURN_VALUE
58

59. Python Bytecode
def maximum(a, b):
if a > b:
return a
else:
return b
import dis
dis.dis(maximum)
1
2
3
4
5
6
7
8
2 0 LOAD_FAST 0 (a)
2 LOAD_FAST 1 (b)
4 COMPARE_OP 4 (>)
6 POP_JUMP_IF_FALSE 12
3 8 LOAD_FAST 0 (a)
10 RETURN_VALUE
5 >> 12 LOAD_FAST 1 (b)
14 RETURN_VALUE
16 LOAD_CONST 0 (None)
18 RETURN_VALUE
59

60. Python Bytecode
def maximum(a, b):
if a > b:
return a
else:
return b
import dis
dis.dis(maximum)
1
2
3
4
5
6
7
8
2 0 LOAD_FAST 0 (a)
2 LOAD_FAST 1 (b)
4 COMPARE_OP 4 (>)
6 POP_JUMP_IF_FALSE 12
3 8 LOAD_FAST 0 (a)
10 RETURN_VALUE
5 >> 12 LOAD_FAST 1 (b)
14 RETURN_VALUE
16 LOAD_CONST 0 (None)
18 RETURN_VALUE
60

61. Python Bytecode
def maximum(a, b):
if a > b:
return a
else:
return b
import dis
dis.dis(maximum)
1
2
3
4
5
6
7
8
2 0 LOAD_FAST 0 (a)
2 LOAD_FAST 1 (b)
4 COMPARE_OP 4 (>)
6 POP_JUMP_IF_FALSE 12
3 8 LOAD_FAST 0 (a)
10 RETURN_VALUE
5 >> 12 LOAD_FAST 1 (b)
14 RETURN_VALUE
16 LOAD_CONST 0 (None)
18 RETURN_VALUE
61

62. Python Bytecode
2 0 LOAD_FAST 0 (a)
2 LOAD_FAST 1 (b)
4 COMPARE_OP 4 (>)
6 POP_JUMP_IF_FALSE 12
3 8 LOAD_FAST 0 (a)
10 RETURN_VALUE
5 >> 12 LOAD_FAST 1 (b)
14 RETURN_VALUE
16 LOAD_CONST 0 (None)
18 RETURN_VALUE
62

63. 2 0 LOAD_FAST 0 (a)
2 LOAD_FAST 1 (b)
4 COMPARE_OP 4 (>)
6 POP_JUMP_IF_FALSE 12
3 8 LOAD_FAST 0 (a)
10 RETURN_VALUE
5 >> 12 LOAD_FAST 1 (b)
14 RETURN_VALUE
16 LOAD_CONST 0 (None)
18 RETURN_VALUE
Python Bytecode
63

64. 2 0 LOAD_FAST 0 (a)
2 LOAD_FAST 1 (b)
4 COMPARE_OP 4 (>)
6 POP_JUMP_IF_FALSE 12
3 8 LOAD_FAST 0 (a)
10 RETURN_VALUE
5 >> 12 LOAD_FAST 1 (b)
14 RETURN_VALUE
16 LOAD_CONST 0 (None)
18 RETURN_VALUE
Python Bytecode
64

65. 2 0 LOAD_FAST 0 (a)
2 LOAD_FAST 1 (b)
4 COMPARE_OP 4 (>)
6 POP_JUMP_IF_FALSE 12
3 8 LOAD_FAST 0 (a)
10 RETURN_VALUE
5 >> 12 LOAD_FAST 1 (b)
14 RETURN_VALUE
16 LOAD_CONST 0 (None)
18 RETURN_VALUE
Python Bytecode
65

66. Bytecode Modification
2 0 LOAD_FAST 0 (a)
2 LOAD_FAST 1 (b)
4 COMPARE_OP 4 (>)
6 POP_JUMP_IF_FALSE 12
3 8 LOAD_FAST 0 (a)
10 RETURN_VALUE
5 >> 12 LOAD_FAST 1 (b)
14 RETURN_VALUE
16 LOAD_CONST 0 (None)
18 RETURN_VALUE
breakpoint()
66

67. Bytecode Modification
• Insert breakpoint’s code
• Update arguments and offsets
67

68. Bytecode Modification
• Insert breakpoint’s code
• Update arguments and offsets
• 200 lines in Python
68

69. Bytecode Modification
2 0 LOAD_FAST 0 (a)
2 LOAD_FAST 1 (b)
4 COMPARE_OP 4 (>)
6 POP_JUMP_IF_FALSE 12
3 8 LOAD_FAST 0 (a)
10 RETURN_VALUE
5 >> 12 LOAD_FAST 1 (b)
14 RETURN_VALUE
16 LOAD_CONST 0 (None)
18 RETURN_VALUE
breakpoint()
?!
69

70. Breakpoint Bytecode
def _stop_at_break():
# a lot of code here
def breakpoint():
_stop_at_break()
1
2
3
4
5
6
7
8
0 LOAD_GLOBAL 0
2 CALL_FUNCTION 0
4 POP_TOP
6 LOAD_CONST 0
8 RETURN_VALUE
70

71. Build Python Debugger
• Breakpoints
• Stepping
71

72. Stepping
• Inserting temporary breakpoint
on every line
• Use old tracing function
72

73. Frame evaluation
debugger is ready
73

74. Example 1
def calculate():
sum = 0
for i in range(10 ** 7):
sum += i
return sum
1
2
3
4
5
74

75. Example 1
75
Run
Tracing
Frame
evaluation
0,80 sec
19,81 sec
0,81 sec

76. Example 1
76
Run
Tracing
Frame
evaluation
0,80 sec
19,81 sec
0,81 sec

77. Example 2
def foo():
pass
def calculate():
sum = 0
for i in range(10 ** 7):
foo()
sum += i
return sum
1
2
3
4
5
6
7
8
9
77

78. Example 2
78
Run
Tracing
Frame
evaluation
1,73 sec
43,58 sec
37,41 sec

79. PEP 523
• Handle evaluation of frames
• Add a new field to code objects
79

80. PEP 523
• Expand PyCodeObject struct
• co_extra - “scratch space” for
the code object
• Mark frames without breakpoints
80

81. Mark Frames Cython
1
2
3
4
5
6
7
8
9
81
cdef frame_eval(PyFrameObject *frame, int exc):
flag = _PyCode_GetExtra(frame.f_code, index)
if flag == NO_BREAKS_IN_FRAME:
return _PyEval_EvalFrameDefault(frame, exc)
# check for breakpoints
...

82. Example 2
82
Run
Tracing
Frame
evaluation
1,73 sec
43,58 sec
1,91 sec

83. PEP 523
• Handle evaluation of frames
• Add a new field to code objects
83

84. Contents
• Tracing debugger
• Python 3.6
• Frame evaluation debugger
• Results
84

85. Contents
• Tracing debugger
• Python 3.6
• Frame evaluation debugger
• Results
85

86. Real Life Example
86

87. Real Life Example
• Included into PyCharm 2017.1
• Works in production
87

88. PyCharm
88
Tracing
w/o Cython
Tracing
with Cython
Frame
evaluation
11,59 sec
5,66 sec
0,28 sec

89. Disadvantages
• More complicated
• Only with CPython
• Only with Python 3.6
89

90. Frame Evaluation
• Let’s move to Python 3.6!
90

91. Frame Evaluation
• Let’s move to Python 3.6!
• Let’s find another use cases
91

92. Use cases
def maximum(a, b):
if a > b:
return a # breakpoint
else:
return b
1
2
3
4
5
6
7
8
breakpoint()
92

93. PEP 523
• Pyjion project
• JIT for Python
93

94. Frame Evaluation
• Let’s move to Python 3.6!
• Let’s find another use cases
94

95. Links
• Prototype: https://github.com/
Elizaveta239/frame-eval
• PyCharm Community Edition source
code
• bytesinsert on PyPi
95

96. • Prototype: https://github.com/
Elizaveta239/frame-eval
• PyCharm Community Edition source
code
• bytesinsert on PyPi
Questions?
96
@lisa_shashkova