Only line 2 has infinite slope
elif ( x4 - x3 == 0 ) : NEW_LINE INDENT m1 = ( y2 - y1 ) / ( x2 - x1 ) ; NEW_LINE if ( m1 == 0 ) : NEW_LINE INDENT return True NEW_LINE DEDENT else : NEW_LINE INDENT return False NEW_LINE DEDENT DEDENT else : NEW_LINE