gotchas 

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Erick Mendonça, Software Engineer, eShares 

mutable default values 

def append_to(element, to=[]): to.append(element) return to Source: http://docs.python-guide.org/en/latest/writing/gotchas/ 

my_list = append_to(12) print(my_list) 

my_list = append_to(42) print(my_list) 

print(my_list) [12, 42] 

mutable function arguments 

def print_sorted(sequence): sequence.sort() print(sequence) my_list = [2, 1, 5, 3] print_sorted(my_list) print(my_list) 

loop updating lists or dicts 

def wrong_loop(): a = list(range(10)) for i in a: a[i+1] = 1 print(a) wrong_loop() 

[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 "", line 1, in File "", line 4, in wrong_loop IndexError: list assignment index out of range 

def wrong_loop_2(): a = list(range(10)) for i in a: if i % 2: a.append(i) print(len(a)) wrong_loop_2() 

∞ 

late binding lambdas 

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/ 

scope 

msg = "Python Rio <3" def print_msg(): print(msg) 

msg = "Python Rio <3" def print_msg(): print(msg) msg = "and that's awesome!" print(msg) 

UnboundLocalError: local variable 'msg' referenced before assignment 

hash() 

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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) 

app-01$ >>> hash('PythonRio') 90314343757402039 app-02$ >>> hash('PythonRio') 5622018667917605056 

Thank you! /@erickmagnus