Added docstring to cg calculation function

calcgc.py

 #!/usr/bin/env python
 import sys
 
+# Python programming course
+# Author: Marc van Driel
+
 def calc_gc_perc(seq):
+    """
+    Calculates the GC% of a sequence string
+    Arguments:
+        - seq - input sequence (string)
+    Returns:
+        - GC percentrage (float)
+    Example:
+    calc_gc_perc('CAGG') # returns 75%
+    """
+
     atc,gcc = 0, 0
     for c in seq.upper():
         if c in ('A', 'T'):
             atc += 1
         elif c in ('C', 'G'):
             gcc += 1
-    return gcc * 100 / (gcc + atc)
+    return gcc * 100.0 / (gcc + atc)
 
 inpseq = sys.argv[1]
 print "the sequence is 'CAGG', and has a GC% of {:.2f}".format( calc_gc_perc(inpseq) )