[PyX-user] path.tangent() and "length" argument

[Lander <la...@we...>]

Hello,

I'd like to thank the PyX developers for this amazing
package.  Congratulations and keep up the good work.

This is for PyX 0.9.  There's something I don't understand
about the tangent method provided by path.  After reading
the documentation, I thought that "length" was an optional
argument, with None as default value.  In this case, a
normalized unit vector would be returned.  Please, consider
the following trivial example:

    cir = path.circle(0, 0, 1)
    tg = cir.tangent(cir.atbegin())

When Python executes the second line, it throws a TypeError
exception.  Here's the traceback:

      File "circles.py", line 9, in ?
        tg = cir.tangent(cir.begin(), None)
      File "[...]/pyx/path.py", line 1164, in tangent
        return self.normpath().tangent(params, length)
      File "[...]/pyx/normpath.py", line 1418, in wrappedmethod
        return method(self, [valueorlist], *args, **kwargs)[0]
      File "[...]/pyx/normpath.py", line 1900, in tangent
        return self._tangent(self._convertparams(params, self.arclentoparam), unit.topt(length))
      File "[...]/pyx/unit.py", line 72, in topt
        return _convert_to(l, "pt")
      File "[...]/pyx/unit.py", line 55, in _convert_to
        l = length(l)       # convert to length instance if necessary
      File "[...]/pyx/unit.py", line 94, in __init__
        l = float(f) * _m[unit or _default_unit]
    TypeError: float() argument must be a string or a number

If you add a length argument different from None, then it
works as expected.

I'm rather new to Python, so don't take the following very
seriously: I've been reading over the code, and I suspect
that neither the tangent method provided by path nor the one
provided by normpath check the type of "length";  therefore,
this length transparently passes as an argument to
unit.topt, and a TypeError is thrown if it's None.

I'd appreciate your helping me understand this.

Cheers.
Lander