Ordered unique elements

unique.py

# An occasional python interview question I have seen is:
#   "How would you make this series unique while preserving order?"
# The standard code looks like this:

def unique2(series):
    result = []
    seen = set()
    for s in series:
        if s not in seen:
            seen.add(s)
            result.append(s)
    return result

# And this works. Here is the extra clever python-y code you can write instead:

def unique(series):
    seen = set()
    return [seen.add(s) or s for s in series if s not in seen]

# The tricky part is that `set.add` returns `None` so if you `or` that with a value,
# you get the value plus the side effect of modifying the `set`. And if you compare the
# disassembled code (using the `dis` module), you can see how much shorter the clever
# code is:

```
from dis import dis
>>> dis(unique)
  2           0 LOAD_GLOBAL              0 (set)
              3 CALL_FUNCTION            0 (0 positional, 0 keyword pair)
              6 STORE_DEREF              0 (a)

  3           9 LOAD_CLOSURE             0 (a)
             12 BUILD_TUPLE              1
             15 LOAD_CONST               1 (<code object <listcomp> at 0x10deb6390, file "<stdin>", line 3>)
             18 LOAD_CONST               2 ('unique3.<locals>.<listcomp>')
             21 MAKE_CLOSURE             0
             24 LOAD_FAST                0 (series)
             27 GET_ITER
             28 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             31 RETURN_VALUE

>>> dis(unique2)
  2           0 BUILD_LIST               0
              3 STORE_FAST               1 (result)

  3           6 LOAD_GLOBAL              0 (set)
              9 CALL_FUNCTION            0 (0 positional, 0 keyword pair)
             12 STORE_FAST               2 (seen)

  4          15 SETUP_LOOP              52 (to 70)
             18 LOAD_FAST                0 (series)
             21 GET_ITER
        >>   22 FOR_ITER                44 (to 69)
             25 STORE_FAST               3 (s)

  5          28 LOAD_FAST                3 (s)
             31 LOAD_FAST                2 (seen)
             34 COMPARE_OP               7 (not in)
             37 POP_JUMP_IF_FALSE       22

  6          40 LOAD_FAST                2 (seen)
             43 LOAD_ATTR                1 (add)
             46 LOAD_FAST                3 (s)
             49 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             52 POP_TOP

  7          53 LOAD_FAST                1 (result)
             56 LOAD_ATTR                2 (append)
             59 LOAD_FAST                3 (s)
             62 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             65 POP_TOP
             66 JUMP_ABSOLUTE           22
        >>   69 POP_BLOCK

  8     >>   70 LOAD_FAST                1 (result)
             73 RETURN_VALUE
```