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

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Jams Cunningham hello

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Core contributor

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

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

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No content

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But first, some broken software!

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

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

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

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Exceptions Errors vs

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Errors

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

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

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

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

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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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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'), )

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

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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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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'), )

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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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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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import sys print(sys.exc_info())

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

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

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

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

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

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

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$ python main.py 2> errors.log

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No content

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$ cat errors.log

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

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

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

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

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Sentry. Gotta get all dat context.

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