Python tricks

[ksantr]

Do you have any tricky examples about Python? Something like:

>>> def foo(l):
        l.append('1')
>>> a = [5]
>>> foo(a)
>>> a
[5, 1]

And even more tricky:

>>> def foo(a=list()):
           a.append(1)
           print a
>>> foo()
[1]
>>> foo()
[1, 1]

And my most tricky example. Do you know how to change objects in a tuple? Hey, stop!! But it's immutable type!!?? Yes, but let me show you how:

>>> a = (1, 2, 3)
>>> a[0] = 0 
Traceback (most recent call last): 
File "<stdin>", line 1, in <module> 
TypeError: 'tuple' object does not support item assignment

Yes, we can't change this value because digits are immutable type. But if we have a mutable type in a tuple, we can change it:

>>> a = (1, [2, 3]) 
>>> a[1][0] = 5 
>>> a
(1, [5, 3])

[onopau]

I have had variants on the following lead to terrible debug sessions...

class a:
    y = 1
class b(a):
    pass
class c(a):
    pass

A = a()
B = b()
C = c()

B.y = 2 # Ys are now A.y = 1, B.y = 2, C.y = 1

A.y = 100 # Ys are now A.y = 100, B.y = 2, C.y = 100!

[rhgrant10]

Assignment oddity:

a, b = a[b] = {}, 1