Here is a Python script:
1 Python 2.7.5+ (default, Sep 19 2013, 13:48:49) 
2 [GCC 4.8.1] on linux2
3 Type "help", "copyright", "credits" or "license" for more information.
4 >>> from cmath import phase
5 >>>
6 >>> print phase(complex(-1.0, 0.0))
7 3.14159265359