Python expression calculator [closed]

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()))

closed as off-topic by Gareth Rees, yuri, Stephen Rauch, Graipher, Donald.McLeanSep 13 '18 at 14:42

This question appears to be off-topic. The users who voted to close gave this specific reason:

• "Code not implemented or not working as intended: Code Review is a community where programmers peer-review your working code to address issues such as security, maintainability, performance, and scalability. We require that the code be working correctly, to the best of the author's knowledge, before proceeding with a review." – Gareth Rees, yuri, Stephen Rauch, Graipher, Donald.McLean
If this question can be reworded to fit the rules in the help center, please edit the question.

• Crashes for 1 + -3 – Martin R Sep 11 '18 at 17:24
• See this answer for some advice on writing an expression evaluator. – Gareth Rees Sep 11 '18 at 18:46
• eval('(1*1)')ValueError: invalid literal for int() with base 10: '1.0' – Gareth Rees Sep 12 '18 at 15:46