Python expression calculator [closed]

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This is a calculator I created. It can take all basic operators, including brackets. I am a beginner with Python so I want to know what can I do better next time I write something similar.

def eval(expression):
 current = 0
 while current < len(expression):
 x = expression[current]
 if x == '(':
 levels = 1
 inside = ''
 while levels:
 current+=1
 x = expression[current]
 if x == ')':
 levels -= 1
 if not levels:
 expression = expression.replace('('+inside+')',str(eval(inside)))
 current -= (len(inside)+2)
 else:
 inside+=')'
 elif x == '(':
 levels+=1
 inside+=x
 else:
 inside+=x
 current+=1
 expressions = list(filter(None,expression.replace('+-','-').replace('-','+-').split('+')))
 ret = 0
 for exp in expressions:
 if exp.find('*') == -1 and exp.find('/') == -1:
 ret+=int(exp)
 continue
 op = '*'
 expret = 1
 exp +='*1'
 while exp.find('*') > -1 or exp.find('/') > -1:
 next_mult = exp.find('*')
 next_div = exp.find('/')
 if next_mult > -1 and (next_mult < next_div or next_div==-1):
 expret = multdiv(expret,op,float(exp[:next_mult]))
 op = '*'
 exp = exp[next_mult+1:]
 elif next_div < next_mult:
 expret = multdiv(expret,op,float(exp[:next_div]))
 op = '/'
 exp = exp[next_div+1:]
 ret += expret
 return ret
def multdiv(num1,op,num2):
 if op == '*':
 return num1*num2
 return num1/num2
print(eval(input()))

• Martin R: Crashes for 1 + -3
• Gareth Rees: See this answer for some advice on writing an expression evaluator.
• Gareth Rees: eval('(1*1)') → ValueError: invalid literal for int() with base 10: '1.0'