Question

In python you can change a list like this:

In [303]: x = [1,2,3,4,5,6]

In [304]: x[x <= 3]+=3

In [305]: x
Out[306]: [4, 2, 3, 4, 5, 6]

I have known about this for some time now, but I don’t think I fully understand whats going on behind the scenes. I would appriciate, if someone would find the time to explain.

In [307]: x = [1,2,3,4,5,6]

In [308]: dis.dis('x[x <= 3]+=3')
          0 SETUP_LOOP      30811 (to 30814)
          3 SLICE+2        
          4 STORE_SUBSCR   
          5 DELETE_SUBSCR  
          6 SLICE+2        
          7 DELETE_SLICE+1 
          8 FOR_ITER        15659 (to 15670)
         11 DELETE_SLICE+1 

In [309]: x
Out[309]: [1, 2, 3, 4, 5, 6]

In [310]: x[x <= 3]+=3

In [311]: x
Out[311]: [4, 2, 3, 4, 5, 6]

In [312]: x<=3
Out[312]: False

In [313]: x[False]+=3

In [314]: x
Out[314]: [7, 2, 3, 4, 5, 6]

Answer 1

x <= 3 is a boolean expression. Since in Python, the boolean type is a subclass of int, the False outcome is interpreted as 0, so the end effect is:

x[0] += 3

Or, demonstrated in a different way:

>>> False == 0
True
>>> True == 1
True
>>> isinstance(False, int)
True

The dis.dis() method doesn’t work with strings; it works with code objects (or something that contains code, such as a function, method, class, or module), or with bytecode. The fact that it seems to be able to decode your string is a happy coincidence; SETUP_LOOP is opcode 120 (the ASCII value for x), the whole string is being interpreted as a set of opcodes and offsets.

Use a function instead:

>>> def foo(): x[x <= 3]+=3
... 
>>> dis.dis(foo)
  1           0 LOAD_GLOBAL              0 (x)
              3 LOAD_GLOBAL              0 (x)
              6 LOAD_CONST               1 (3)
              9 COMPARE_OP               1 (<=)
             12 DUP_TOPX                 2
             15 BINARY_SUBSCR       
             16 LOAD_CONST               1 (3)
             19 INPLACE_ADD         
             20 ROT_THREE           
             21 STORE_SUBSCR        
             22 LOAD_CONST               0 (None)
             25 RETURN_VALUE