Python base converter

base_n.py

class MathExc(Exception):
	def __init__(self,msg):
		print "\nError: "+str(msg)+" !"

class Converter (object):
	def __init__(self,base):
		self._BaseN_alpha=None 
		self._BaseN_num=None
		self._BaseN_base=None 
		base=int(base)
		if base <=0:
			raise MathExc('Base must be greater than 0')
		self._BaseN_base=base
		if base==2:
			self._BaseN_alpha='01'
		elif base==8:
			self._BaseN_alpha='01234567'
		elif base==16:
			self._BaseN_alpha='0123456789abcdef'
		elif base==62:
			from string import letters, digits
			self._BaseN_alpha=digits+letters
		elif base==52:
			from string import letters
			self._BaseN_alpha=letters
		else:
			raise MathExc('Unsupported base')
			
	def _FromBase62(self,String):
		a=len(String)-1
		num=long(0)
		for let in String:
			num+=(self._BaseN_alpha.find(let)+1)*(self._BaseN_base**a)
			a-=1
		return num

	def _ToBase62(self,num):
		str=''
		while num:
			str+=self._BaseN_alpha[(num-1)%self._BaseN_base]
			num=(num-1)/self._BaseN_base
		return ''.join(list(reversed(str)))		
		
	def ToBase(self,num):
		try:
			num=long(num)
		except Exception as e:
			raise MathExc(str(e))
		if self._BaseN_base==62:
			return self._ToBase62(num)
		str=''
		self._BaseN_num=int(num)
		if not num:
			return self._BaseN_alpha[0]
		while num>0:
			str+=self._BaseN_alpha[num%self._BaseN_base]
			num/=self._BaseN_base
		return ''.join(list(reversed(str)))
		
	def FromBase(self,String):
		try:
			String=str(String)
		except Exception as e:
			raise MathExc(str(e))
		if self._BaseN_base==62:
			return self._FromBase62(String)
			
		return int(String, base=self._BaseN_base)
		a=len(String)
		num=long(0)-1
		for let in String:
			num+=(self._BaseN_alpha.find(let)+1)*(self._BaseN_base**a)
			a-=1
		return num

for	i in xrange(0,100):
	a=Converter(16).ToBase(i)
	b=long(Converter(16).FromBase(a))
	assert b==i