compton-convgen: Misc: Clean up

compton-convgen: Misc: Clean up. The commit brings no change to the
functionality of the script.

 - Partially fix PEP 8 compliance:

   - Place imports on separate lines.

   - Replace leading tabs with 4 spaces.

   - Add docstrings to classes and functions.

   - Surround top-level function and class definitions with two blank
     lines.

   - Remove spaces around keyword arguments.

   - Move all statements to separate lines.

   - Break some long lines into several lines.

 - Remove trailing semicolons after statements.

 - CGError: Use functionality from the base class Exception to store the
   description, instead of the custom logic.

 - CGInternal: Remove, as it is unused.

 - Hide the internal function gen_invalid() and args_readfactors() by
   prefixing their names with an underscore.

 - Move the module-level command-line handling code to two new
   functions, _main() and _parse_args(), and only execute if running in
   the main scope.

bin/compton-convgen.py

@@ -2,16 +2,21 @@
 # -*- coding: utf-8 -*-
 # vim:fileencoding=utf-8
 
-import math, argparse
+import math
+import argparse
+
 
 class CGError(Exception):
-	def __init__(self, value):
-		self.value = value
-	def __str__(self):
-		return repr(self.value)
+    '''An error in the convolution kernel generator.'''
+    def __init__(self, desc):
+        super().__init__(desc)
+
+
+class CGBadArg(CGError):
+    '''An exception indicating an invalid argument has been passed to the
+    convolution kernel generator.'''
+    pass
 
-class CGBadArg(CGError): pass
-class CGInternal(CGError): pass
 
 def mbuild(width, height):
     """Build a NxN matrix filled with 0."""
@@ -22,14 +27,16 @@ def mbuild(width, height):
             result[i].append(0.0)
     return result
 
+
 def mdump(matrix):
     """Dump a matrix in natural format."""
     for col in matrix:
-		print("[ ", end = '');
+        print("[ ", end='')
         for ele in col:
             print(format(ele, "13.6g") + ", ", end=" ")
         print("],")
 
+
 def mdumpcompton(matrix):
     """Dump a matrix in compton's format."""
     width = len(matrix[0])
@@ -38,21 +45,24 @@ def mdumpcompton(matrix):
     for i in range(height):
         for j in range(width):
             if int(height / 2) == i and int(width / 2) == j:
-				continue;
+                continue
             print(format(matrix[i][j], ".6f"), end=",")
     print()
 
+
 def mnormalize(matrix):
     """Scale a matrix according to the value in the center."""
     width = len(matrix[0])
     height = len(matrix)
     factor = 1.0 / matrix[int(height / 2)][int(width / 2)]
-	if 1.0 == factor: return matrix
+    if 1.0 == factor:
+        return matrix
     for i in range(height):
         for j in range(width):
             matrix[i][j] *= factor
     return matrix
 
+
 def mmirror4(matrix):
     """Do a 4-way mirroring on a matrix from top-left corner."""
     width = len(matrix[0])
@@ -64,6 +74,7 @@ def mmirror4(matrix):
             matrix[i][j] = matrix[x][y]
     return matrix
 
+
 def gen_gaussian(width, height, factors):
     """Build a Gaussian blur kernel."""
 
@@ -76,14 +87,17 @@ def gen_gaussian(width, height, factors):
     result = mbuild(size, size)
     for i in range(int(size / 2) + 1):
         for j in range(int(size / 2) + 1):
-			diffx = i - int(size / 2);
-			diffy = j - int(size / 2);
-			result[i][j] = 1.0 / (2 * math.pi * sigma) * pow(math.e, - (diffx * diffx + diffy * diffy) / (2 * sigma * sigma))
+            diffx = i - int(size / 2)
+            diffy = j - int(size / 2)
+            result[i][j] = 1.0 / (2 * math.pi * sigma) \
+                * pow(math.e, - (diffx * diffx + diffy * diffy) \
+                / (2 * sigma * sigma))
     mnormalize(result)
     mmirror4(result)
 
     return result
 
+
 def gen_box(width, height, factors):
     """Build a box blur kernel."""
     result = mbuild(width, height)
@@ -92,10 +106,13 @@ def gen_box(width, height, factors):
             result[i][j] = 1.0
     return result
 
-def gen_invalid(width, height, factors):
+
+def _gen_invalid(width, height, factors):
+    '''Handle a convolution kernel generation request of an unrecognized type.'''
     raise CGBadArg("Unknown kernel type.")
 
-def args_readfactors(lst):
+
+def _args_readfactors(lst):
     """Parse the factor arguments."""
     factors = dict()
     if lst:
@@ -108,13 +125,21 @@ def args_readfactors(lst):
             factors[res[0]] = float(res[2])
     return factors
 
+
+def _parse_args():
+    '''Parse the command-line arguments.'''
+
     parser = argparse.ArgumentParser(description='Build a convolution kernel.')
     parser.add_argument('type', help='Type of convolution kernel. May be "gaussian" (factor sigma = 0.84089642) or "box".')
     parser.add_argument('width', type=int, help='Width of convolution kernel. Must be an odd number.')
     parser.add_argument('height', nargs='?', type=int, help='Height of convolution kernel. Must be an odd number. Equals to width if omitted.')
     parser.add_argument('-f', '--factor', nargs='+', help='Factors of the convolution kernel, in name=value format.')
     parser.add_argument('--dump-compton', action='store_true', help='Dump in compton format.')
-args = parser.parse_args()
+    return parser.parse_args()
+
+
+def _main():
+    args = _parse_args()
 
     width = args.width
     height = args.height
@@ -122,11 +147,15 @@ if not height:
         height = width
     if not (width > 0 and height > 0):
         raise CGBadArg("Invalid width/height.")
-factors = args_readfactors(args.factor)
+    factors = _args_readfactors(args.factor)
 
     funcs = dict(gaussian=gen_gaussian, box=gen_box)
-matrix = (funcs.get(args.type, gen_invalid))(width, height, factors)
+    matrix = (funcs.get(args.type, _gen_invalid))(width, height, factors)
     if args.dump_compton:
         mdumpcompton(matrix)
     else:
         mdump(matrix)
+
+
+if __name__ == '__main__':
+    _main()