Everything is broken, and I don't know why. Python edition.

SF Python
October 12th 2016

1. Everything is broken, and I don’t know why. Python edition SF Python October 12th 2016 Matt Robenolt
2. Jams Cunningham hello
3. Core contributor
4. If an exception happens in production, and nobody sees the logs, did it really happen? — Taylor Swift “ ”
5. Damn Stebe, it works for me. gtfo my inbox. “ ” — Josh
7. But first, some broken software!
8. #include <stdio.h> int main () { FILE *fp; char buf[100]; fp = fopen("foo.txt", "r"); fgets(buf, 100, fp); fclose(fp); } Errors in: C
9. #include <stdio.h> int main () { FILE *fp; char buf[100]; fp = fopen("foo.txt", "r"); fgets(buf, 100, fp); fclose(fp); } Errors in: C [1] 53482 segmentation fault ./a.out
10. #include <stdio.h> int main () { FILE *fp; char buf[100]; fp = fopen("foo.txt", “r"); if (fp != NULL) { fgets(buf, 100, fp); fclose(fp); } } Errors in: C
11. package main import "os" func main() { buf := make([]byte, 100) fp, _ := os.Open("foo.txt") fp.Read(buf) fp.Close() } Errors in: Go
12. package main import "os" func main() { buf := make([]byte, 100) fp, _ := os.Open("foo.txt") fp.Read(buf) fp.Close() } Errors in: Go jk nothing happens
13. package main import "os" func main() { buf := make([]byte, 100) fp, err := os.Open(“foo.txt") if err == nil { fp.Read(buf) fp.Close() } } Errors in: Go
14. use std::fs::File; use std::io::Read; fn main() { let mut fp = File::open("foo.txt").unwrap(); let mut buf = [0; 100]; fp.read_exact(&mut buf); drop(fp); } Errors in: Rust
15. use std::fs::File; use std::io::Read; fn main() { let mut fp = File::open("foo.txt").unwrap(); let mut buf = [0; 100]; fp.read_exact(&mut buf); drop(fp); } Errors in: Rust thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 2, message: "No such file or directory" } }', ../src/ libcore/result.rs:788 note: Run with `RUST_BACKTRACE=1` for a backtrace.
16. use std::fs::File; use std::io::Read; fn main() { match File::open("foo.txt") { Ok(mut fp) => { let mut buf = [0; 100]; fp.read_exact(&mut buf); drop(fp); }, Err(_) => { // }, } } Errors in: Rust
17. fp = open('foo.txt', 'r') buf = fp.read(100) fp.close() Errors in: Python
18. fp = open('foo.txt', 'r') buf = fp.read(100) fp.close() Errors in: Python Traceback (most recent call last): File "main.py", line 1, in <module> fp = open('foo.txt', 'r') IOError: [Errno 2] No such file or directory: 'foo.txt'
19. try: fp = open('foo.txt', 'r') except IOError: pass else: buf = fp.read(100) fp.close() Errors in: Python
20. try: fp = open('foo.txt', 'r') except IOError: pass else: buf = fp.read(100) fp.close() Errors in: Python Exceptions
21. Exceptions Errors vs
22. Errors
23. Errors Non-fatal, multiple may exist within a call stack at the same time, generally returned as a value.
24. Exceptions Fatal, must be handled, only one exists within a call stack, raised to abort current execution.
25. Exceptions Fatal, must be handled, only one exists within a call stack, raised to abort current execution.
26. Traceback (most recent call last): File "main.py", line 1, in <module> fp = open('foo.txt', 'r') IOError: [Errno 2] No such file or directory: 'foo.txt'
27. Traceback (most recent call last): File "main.py", line 1, in <module> fp = open('foo.txt', 'r') IOError: [Errno 2] No such file or directory: 'foo.txt' So what do we do with this thing?
28. Traceback (most recent call last): File "main.py", line 1, in <module> fp = open('foo.txt', 'r') IOError: [Errno 2] No such file or directory: 'foo.txt' So what do we do with this thing? …and where does it come from?
29. fp = open('foo.txt', 'r') buf = fp.read(100) fp.close()
30. import sys def excepthook(type, value, tb): print(type, value, tb) sys.excepthook = excepthook fp = open('foo.txt', 'r') buf = fp.read(100) fp.close()
31. import sys def excepthook(type, value, tb): print(type, value, tb) sys.excepthook = excepthook fp = open('foo.txt', 'r') buf = fp.read(100) fp.close() (<type 'exceptions.IOError'>, IOError(2, 'No such file or directory'), <traceback object at 0x106810ea8>)
32. import sys def excepthook(type, value, tb): import traceback traceback.print_exception(type, value, tb) sys.excepthook = excepthook fp = open('foo.txt', 'r') buf = fp.read(100) fp.close()
33. import sys def excepthook(type, value, tb): import traceback traceback.print_exception(type, value, tb) sys.excepthook = excepthook fp = open('foo.txt', 'r') buf = fp.read(100) fp.close() Traceback (most recent call last): File "main.py", line 9, in <module> fp = open('foo.txt', 'r') IOError: [Errno 2] No such file or directory: 'foo.txt'
34. fp = open('foo.txt', 'r') buf = fp.read(100) fp.close()
35. try: fp = open('foo.txt', 'r') except IOError: import sys print(sys.exc_info()) else: buf = fp.read(100) fp.close()
36. try: fp = open('foo.txt', 'r') except IOError: import sys print(sys.exc_info()) else: buf = fp.read(100) fp.close() (<type 'exceptions.IOError'>, IOError(2, 'No such file or directory'), <traceback object at 0x106810ea8>)
37. try: fp = open('foo.txt', 'r') except IOError: import sys, traceback traceback.print_exception(*sys.exc_info()) else: buf = fp.read(100) fp.close()
38. try: fp = open('foo.txt', ‘r') except IOError: import traceback traceback.print_exc() else: buf = fp.read(100) fp.close() Traceback (most recent call last): File "main.py", line 2, in <module> fp = open('foo.txt', 'r') IOError: [Errno 2] No such file or directory: 'foo.txt'
39. import sys print(sys.exc_info())
40. import sys print(sys.exc_info()) (None, None, None)
41. import sys try: raise Exception() except Exception: pass print(sys.exc_info())
42. import sys try: raise Exception() except Exception: pass print(sys.exc_info()) (None, None, None)
43. try: raise Exception() except Exception: import sys print(sys.exc_info())
44. try: raise Exception() except Exception: import sys print(sys.exc_info()) (<type 'exceptions.Exception'>, Exception(), <traceback object at 0x10ffbec20>)
45. try: raise Exception() except Exception: import traceback, sys traceback.print_exc(file=sys.stderr)
46. $ python main.py 2> errors.log
48. $ cat errors.log
49. $ cat errors.log raise Exception() Exception Traceback (most recent call last): File "main.py", line 3, in <module> raise Exception() Exception Traceback (most recent call last): File "main.py", line 3, in <module> raise Exception() Exception Traceback (most recent call last): File "main.py", line 3, in <module> raise Exception() Exception Traceback (most recent call last): File "main.py", line 3, in <module>
50. $ cat errors.log Traceback (most recent call last): File "main.py", line 4, in <module> my_thing = things[key] KeyError: 'thing1' Traceback (most recent call last): File "main.py", line 4, in <module> my_thing = things[key] KeyError: 'thing2' Traceback (most recent call last): File "main.py", line 4, in <module> my_thing = things[key] KeyError: 'thing3' Traceback (most recent call last): File "main.py", line 4, in <module> my_thing = things[key] KeyError: 'thing4'
51. $ cat errors.log Traceback (most recent call last): File "main.py", line 4, in <module> my_thing = things[key] KeyError: 'thing1' Traceback (most recent call last): File "main.py", line 4, in <module> my_thing = things[key] KeyError: 'thing2' Traceback (most recent call last): File "main.py", line 4, in <module> my_thing = things[key] KeyError: 'thing3' Traceback (most recent call last): File "main.py", line 4, in <module> my_thing = things[key] KeyError: 'thing4'
52. Traceback (most recent call last): File "main.py", line 4, in <module> my_thing = things[key] KeyError: 'thing1' Traceback (most recent call last): File "main.py", line 4, in <module> my_thing = things[key] KeyError: 'thing2' Traceback (most recent call last): File "main.py", line 4, in <module> my_thing = things[key] KeyError: 'thing3' Traceback (most recent call last): File "main.py", line 4, in <module> my_thing = things[key] KeyError: 'thing4'
53. Damn Stebe, that variable should be right. You must have messed it up. “ ” — Darrell
54. things = {} key = sys.argv[1] try: my_thing = things[key] except KeyError: import traceback traceback.print_exc() print(globals(), locals())
55. things = {} key = sys.argv[1] try: my_thing = things[key] except KeyError: import traceback traceback.print_exc() print(globals(), locals())
56. things = {} key = sys.argv[1] try: my_thing = things[key] except KeyError: import traceback traceback.print_exc() print(globals(), locals()) Traceback (most recent call last): File "main.py", line 5, in <module> things[key] KeyError: 'thing' ({'things': {},'key': 'thing'}, {'things': {}, 'key': 'thing'})
57. things = {} key = sys.argv[1] try: my_thing = things[key] except KeyError: import traceback traceback.print_exc() print(globals(), locals()) Traceback (most recent call last): File "main.py", line 5, in <module> things[key] KeyError: 'thing' ({'things': {},'key': 'thing'}, {'things': {}, 'key': 'thing'})
58. things = {} key = sys.argv[1] try: my_thing = things[key] except KeyError: import traceback, json, sys json.dump({ 'exc': traceback.format_exc(), 'globals': globals(), 'locals': locals(), }, sys.stderr)
59. things = [0, 1, 2] def get_random_thing(n): return get_thing(randint(0, n)) def get_thing(index): try: return things[index] except IndexError: print(locals()) get_random_thing(10)
60. things = [0, 1, 2] def get_random_thing(n): return get_thing(randint(0, n)) def get_thing(index): try: return things[index] except IndexError: print(locals()) get_random_thing(10) {'index': 8}
61. def debug(): import sys tb = sys.exc_info()[2] idx = 0 while tb: idx += 1 frame = tb.tb_frame code = frame.f_code print({ 'line': tb.tb_lineno, 'locals': frame.f_locals, 'function': code.co_name, }) tb = tb.tb_next try: get_random_thing(10) except Exception: debug()
62. def debug(): import sys tb = sys.exc_info()[2] idx = 0 while tb: idx += 1 frame = tb.tb_frame code = frame.f_code print({ 'line': tb.tb_lineno, 'locals': frame.f_locals, 'function': code.co_name, }) tb = tb.tb_next try: get_random_thing(10) except Exception: debug() {'function': ‘<module>', 'line': 31, 'locals': {'things': [0, 1, 2]}} {'function': 'get_random_thing', 'line': 7, 'locals': {'n': 4}} {'function': 'get_thing', 'line': 11, 'locals': {'index': 4}}
63. Hot damn Stebe, thanks for the information. Lemme fix that bug for you right away. “ ”
64. Sentry. Gotta get all dat context.
65. Questions? I may or may not have answers. @mattrobenolt github.com/mattrobenolt sentry.io github.com/getsentry/sentry