Python config saver and loader

import json

class Config:
    def __init__(self, file="config.pycfg"):
        self.file = file

    def get(self, key):
        f = open(self.file, "r")
        data = f.read()
        f.close()
        for line in data.splitlines():
            sp = line.split('=')
            if sp[0] == key:
                return sp[1]

        return None

    def add(self,key,amount=1):
        d = int(self.get(key))
        if d:
            self.set(key, d+amount)
        else:
            self.set(key, amount)


    def set(self, key, value):

        f = open(self.file, "r+")
        data = f.read()
        f.close()

        f = open(self.file, "w+")

        found_line = None

        for line in data.splitlines():
            sp = line.split('=')
            if sp[0] == key:
                found_line = line

        new = "%s=%s" % (key, value)
        if found_line:
            data = data.replace(found_line, new)
        else:
            data = "%s\n%s" % (data, new)

        f.write(data)
        f.close()