This patch adds the familiar "Arg" function from
complex analysis to the cmath module, though it's
called "arg" here for consistency with the other names.
Along with the built-in abs function this makes
polar/rectangular coordinate conversions trivial:

  z = complex(x,y)
  r, theta = abs(z), arg(z)
  
  z = r * exp(1j * theta)
  x, y = z.real, z.imag

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I forgot to mention this patch is relative to version 2.2.1.

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For Tim to pronounce.

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It's hard to know what to say -- since it's really just an 
alias for math.atan2, it's hard to get excited about.  
Might it be even more convenient to add

cmath.polar(complex) -> (abs, arg)
cmath.rect(abs, arg) -> complex

functions?  That is, what do you *really* want <wink>?

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You have a good point; the functions you suggest *are* a lot
closer to what is needed in practice, but at the same time
something just bothers me about a function I learned about
in the first week of complex analysis not being in the
complex math module--kind of like saying you don't need tan
since it's just really just sin/cos.

How would you feel about adding all three?

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It takes only a couple of minutes to get used to the existing 
functions, so I don't think any of the three should be added.

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Since Raymond's analysis is uncommented, I reject this 
patch. John, if you can find a rationale for this change based 
on practicality, not purity, feel free to post it here - we will still 
see your comments.