Python Gotchas

Presented on PythonRio meetup.

Transcript

1. gotchas
3. Erick Mendonça, Software Engineer, eShares
4. mutable default values
5. def append_to(element, to=[]): to.append(element) return to Source: http://docs.python-guide.org/en/latest/writing/gotchas/
6. my_list = append_to(12) print(my_list)
7. my_list = append_to(42) print(my_list)
8. print(my_list) [12, 42]
9. mutable function arguments
10. def print_sorted(sequence): sequence.sort() print(sequence) my_list = [2, 1, 5, 3] print_sorted(my_list) print(my_list)
11. loop updating lists or dicts
12. def wrong_loop(): a = list(range(10)) for i in a: a[i+1] = 1 print(a) wrong_loop()
13. [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] [0, 1, 1, 3, 4, 5, 6, 7, 8, 9] [0, 1, 1, 3, 4, 5, 6, 7, 8, 9] [0, 1, 1, 3, 1, 5, 6, 7, 8, 9] [0, 1, 1, 3, 1, 5, 6, 7, 8, 9] [0, 1, 1, 3, 1, 5, 1, 7, 8, 9] [0, 1, 1, 3, 1, 5, 1, 7, 8, 9] [0, 1, 1, 3, 1, 5, 1, 7, 1, 9] [0, 1, 1, 3, 1, 5, 1, 7, 1, 9] Traceback (most recent call last): File "<stdin>", line 1, in <module> File "<stdin>", line 4, in wrong_loop IndexError: list assignment index out of range
14. def wrong_loop_2(): a = list(range(10)) for i in a: if i % 2: a.append(i) print(len(a)) wrong_loop_2()
15. ∞
16. late binding lambdas
17. def create_multipliers(): return [ lambda x : i * x for i in range(5) ] for multiplier in create_multipliers(): print multiplier(2) Source: http://docs.python-guide.org/en/latest/writing/gotchas/
18. scope
19. msg = "Python Rio <3" def print_msg(): print(msg)
20. msg = "Python Rio <3" def print_msg(): print(msg) msg = "and that's awesome!" print(msg)
21. UnboundLocalError: local variable 'msg' referenced before assignment
22. hash()
24. data = { 'full_name': 'Bruce Banner', 'birth_date': '1962-05-01', # a lot of other data } cache_id = hash(data['full_name']) | hash(data['birth_date']) cache.set(cache_id, data)
25. app-01$ >>> hash('PythonRio') 90314343757402039 app-02$ >>> hash('PythonRio') 5622018667917605056
26. Thank you! /@erickmagnus