[Tutor] Trickier exercise: convert string to complex number

[Angus Rodgers]

Wesley Chun, /Core Python Programming/, Ex. 6-13:

 "[...] An atoc() was never implemented in the string module, so
 that is your task here.  atoc() takes a single string as input,
 a string representation of a complex number [...] and returns
 the equivalent complex number object with the given value [...]"

<http://python.pastebin.com/d6f7c679f>   (retention: 1 day)
<http://python.pastebin.com/d2ea157ff>   (retention: 1 day)
(helper functions for user input)

The main functions are short enough to post here, I think, but
there are many explanatory and apologetic comments in the first
file above; also, I would welcome (with a grimace!) any comments
as to whether I am developing a cranky or baroque style (as I'm
working very much on my own, apart from whatever feedback I can
get here), and such misdemeanours are more likely to be noticed
in the second (and longer) of the two files above.

from string import whitespace

def no_spaces(str):
    return ''.join([ch for ch in str if ch not in whitespace])

def atoc(num):
    """Convert string representation to complex number."""
    num = no_spaces(num)
    n = len(num)
    if not n:
        raise ValueError
    # Ignore first character
    for i, ch in enumerate(num[1:]):
        if ch == 'j':
            # Must be end of string, else invalid
            if i != n - 2:
                raise ValueError
            return complex(0.0, float(num[:-1]))
        if ch in '+-' and num[i] not in 'eE':
            return complex(float(num[:i + 1]),
                           float(num[i + 1:-1]))
    return complex(float(num), 0.0)

-- 
Angus Rodgers