How do I increment base converted numbers

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Python

I am trying make a counter for base 4 numbers the base conversion is successful but when it comes to making a counter it gives me an error, can someone help?

What I have tried:

x=7
i=1
n=5
base=4
def toStr(n,base):
   convertString = "0123"
   if n < base:
      return convertString[n]
   else:
      return toStr(n//base,base) + convertString[n%base]
   return tostr(x//base,base)+convertstring[x%base]
   return tostr(i//base,base)+convertstring[i%base]

def counter(low, high):
    current = low
    while current <= high:
        yield current
        current += 1

for c in counter(toStr(n,base), toStr(x,base)):
    print(c)

print(toStr(n,base))
print(toStr(x,base))
print(toStr(i,base))

x=7
i=1
n=5
base=4
def toStr(n,base):
   convertString = "0123"
   if n < base:
      return convertString[n]
   else:
      return toStr(n//base,base) + convertString[n%base]
   return tostr(x//base,base)+convertstring[x%base]
   return tostr(i//base,base)+convertstring[i%base]

def counter(low, high):
    current = low
    while current <= high:
        yield current
        current += 1

for c in counter(toStr(n,base), toStr(x,base)):
    print(c)