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

SF Python
October 12th 2016

Transcript

1. Everything is broken,
   and I don’t know why.
   Python edition
   SF Python
   October 12th 2016
   Matt Robenolt
   

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2. Jams Cunningham
   hello
   

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3. Core contributor
   

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4. If an exception happens in
   production, and nobody sees
   the logs, did it really happen?
   — Taylor Swift
   “
   ”
   

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5. Damn Stebe, it works for me.
   gtfo my inbox.
   “
   ”
   — Josh
   

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7. But first,
   some broken software!
   

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8. #include
   int main () {
   FILE *fp;
   char buf[100];
   fp = fopen("foo.txt", "r");
   fgets(buf, 100, fp);
   fclose(fp);
   }
   Errors in: C
   

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9. #include
   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
   

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10. #include
    int main () {
    FILE *fp;
    char buf[100];
    fp = fopen("foo.txt", “r");
    if (fp != NULL) {
    fgets(buf, 100, fp);
    fclose(fp);
    }
    }
    Errors in: C
    

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11. package main
    import "os"
    func main() {
    buf := make([]byte, 100)
    fp, _ := os.Open("foo.txt")
    fp.Read(buf)
    fp.Close()
    }
    Errors in: Go
    

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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
    

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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
    

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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
    

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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.
    

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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
    

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17. fp = open('foo.txt', 'r')
    buf = fp.read(100)
    fp.close()
    Errors in: Python
    

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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
    fp = open('foo.txt', 'r')
    IOError: [Errno 2] No such file or directory:
    'foo.txt'
    

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19. try:
    fp = open('foo.txt', 'r')
    except IOError:
    pass
    else:
    buf = fp.read(100)
    fp.close()
    Errors in: Python
    

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20. try:
    fp = open('foo.txt', 'r')
    except IOError:
    pass
    else:
    buf = fp.read(100)
    fp.close()
    Errors in: Python
    Exceptions
    

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21. Exceptions
    Errors
    vs
    

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22. Errors
    

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23. Errors
    Non-fatal, multiple may exist within
    a call stack at the same time,
    generally returned as a value.
    

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24. Exceptions
    Fatal, must be handled, only one
    exists within a call stack, raised to
    abort current execution.
    

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25. Exceptions
    Fatal, must be handled, only one
    exists within a call stack, raised to
    abort current execution.
    

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26. Traceback (most recent call last):
    File "main.py", line 1, in
    fp = open('foo.txt', 'r')
    IOError: [Errno 2] No such file or directory:
    'foo.txt'
    

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27. Traceback (most recent call last):
    File "main.py", line 1, in
    fp = open('foo.txt', 'r')
    IOError: [Errno 2] No such file or directory:
    'foo.txt'
    So what do we do with this thing?
    

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28. Traceback (most recent call last):
    File "main.py", line 1, in
    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?
    

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29. fp = open('foo.txt', 'r')
    buf = fp.read(100)
    fp.close()
    

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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()
    

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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()
    (, IOError(2, 'No such
    file or directory'), 0x106810ea8>)
    

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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()
    

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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
    fp = open('foo.txt', 'r')
    IOError: [Errno 2] No such file or directory:
    'foo.txt'
    

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34. fp = open('foo.txt', 'r')
    buf = fp.read(100)
    fp.close()
    

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35. try:
    fp = open('foo.txt', 'r')
    except IOError:
    import sys
    print(sys.exc_info())
    else:
    buf = fp.read(100)
    fp.close()
    

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36. try:
    fp = open('foo.txt', 'r')
    except IOError:
    import sys
    print(sys.exc_info())
    else:
    buf = fp.read(100)
    fp.close()
    (, IOError(2, 'No such
    file or directory'), 0x106810ea8>)
    

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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()
    

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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
    fp = open('foo.txt', 'r')
    IOError: [Errno 2] No such file or directory:
    'foo.txt'
    

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39. import sys
    print(sys.exc_info())
    

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40. import sys
    print(sys.exc_info())
    (None, None, None)
    

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41. import sys
    try:
    raise Exception()
    except Exception:
    pass
    print(sys.exc_info())
    

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42. import sys
    try:
    raise Exception()
    except Exception:
    pass
    print(sys.exc_info())
    (None, None, None)
    

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43. try:
    raise Exception()
    except Exception:
    import sys
    print(sys.exc_info())
    

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44. try:
    raise Exception()
    except Exception:
    import sys
    print(sys.exc_info())
    (, Exception(),
    )
    

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45. try:
    raise Exception()
    except Exception:
    import traceback, sys
    traceback.print_exc(file=sys.stderr)
    

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46. $ python main.py 2> errors.log
    

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48. $ cat errors.log
    

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49. $ cat errors.log
    raise Exception()
    Exception
    Traceback (most recent call last):
    File "main.py", line 3, in
    raise Exception()
    Exception
    Traceback (most recent call last):
    File "main.py", line 3, in
    raise Exception()
    Exception
    Traceback (most recent call last):
    File "main.py", line 3, in
    raise Exception()
    Exception
    Traceback (most recent call last):
    File "main.py", line 3, in
    

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50. $ cat errors.log
    Traceback (most recent call last):
    File "main.py", line 4, in
    my_thing = things[key]
    KeyError: 'thing1'
    Traceback (most recent call last):
    File "main.py", line 4, in
    my_thing = things[key]
    KeyError: 'thing2'
    Traceback (most recent call last):
    File "main.py", line 4, in
    my_thing = things[key]
    KeyError: 'thing3'
    Traceback (most recent call last):
    File "main.py", line 4, in
    my_thing = things[key]
    KeyError: 'thing4'
    

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51. $ cat errors.log
    Traceback (most recent call last):
    File "main.py", line 4, in
    my_thing = things[key]
    KeyError: 'thing1'
    Traceback (most recent call last):
    File "main.py", line 4, in
    my_thing = things[key]
    KeyError: 'thing2'
    Traceback (most recent call last):
    File "main.py", line 4, in
    my_thing = things[key]
    KeyError: 'thing3'
    Traceback (most recent call last):
    File "main.py", line 4, in
    my_thing = things[key]
    KeyError: 'thing4'
    

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52. Traceback (most recent call last):
    File "main.py", line 4, in
    my_thing = things[key]
    KeyError: 'thing1'
    Traceback (most recent call last):
    File "main.py", line 4, in
    my_thing = things[key]
    KeyError: 'thing2'
    Traceback (most recent call last):
    File "main.py", line 4, in
    my_thing = things[key]
    KeyError: 'thing3'
    Traceback (most recent call last):
    File "main.py", line 4, in
    my_thing = things[key]
    KeyError: 'thing4'
    

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53. Damn Stebe, that
    variable should be
    right. You must
    have messed it up.
    “
    ”
    — Darrell
    

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54. things = {}
    key = sys.argv[1]
    try:
    my_thing = things[key]
    except KeyError:
    import traceback
    traceback.print_exc()
    print(globals(), locals())
    

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55. things = {}
    key = sys.argv[1]
    try:
    my_thing = things[key]
    except KeyError:
    import traceback
    traceback.print_exc()
    print(globals(), locals())
    

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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
    things[key]
    KeyError: 'thing'
    ({'things': {},'key': 'thing'}, {'things':
    {}, 'key': 'thing'})
    

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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
    things[key]
    KeyError: 'thing'
    ({'things': {},'key': 'thing'}, {'things':
    {}, 'key': 'thing'})
    

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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)
    

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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)
    

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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}
    

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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()
    

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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': ‘',
    'line': 31,
    'locals': {'things': [0, 1, 2]}}
    {'function': 'get_random_thing',
    'line': 7,
    'locals': {'n': 4}}
    {'function': 'get_thing',
    'line': 11,
    'locals': {'index': 4}}
    

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63. Hot damn Stebe,
    thanks for the
    information. Lemme fix
    that bug for you right
    away.
    “
    ”
    

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64. Sentry.
    Gotta get all dat context.
    

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65. Questions?
    I may or may not have answers.
    @mattrobenolt
    github.com/mattrobenolt
    sentry.io
    github.com/getsentry/sentry
    

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