string - TypeError: an integer is required (got type str) on function join - python -

i want store foto data in file in python. strange characters in file files not open properly. i'm trying remove data array before save in file:

def save_foto(self):     """ save foto data file """     self.data_aux = ''     last = 0     self.data_list = list(self.vfoto)     in range(0,len(self.vfoto)):         if(self.vfoto[i]=='\x90' , self.vfoto[i+1]=='\x00' , self.vfoto[i+2]=='\x00' , self.vfoto[i+3]=='\x00'            , self.vfoto[i+4]=='\x00' , self.vfoto[i+5]=='\x00' , self.vfoto[i+6]=='\x00' , self.vfoto[i+7]=='\x00'            , self.vfoto[i+8]=='\x00' , self.vfoto[i+9]=='\x00'):              aux1=''.join(map(chr,self.data_list[last:i]))             self.data_aux = self.data_aux+aux1             i=i+10             last=i 

but error

"typeerror: integer required (got type str)" on line aux1=''.join(map(chr,self.data_list[last:i])).

can 1 me , explain me whats goin on? in advance.

i suspect problem comes not using binary mode when reading , writing file. see this question basic read/write binary file in python.