Leetcode: String to Integer (atoi) @Python

Cheating with int():

class Solution:
    # @return an integer
    def atoi(self, str):
        str = str.strip()
        newStr = []
        for i in range(len(str)):
            if '0' <= str[i] <= '9' or (str[i] in ('+', '-') and i == 0):
                newStr.append(str[i])
            else:
                break
        if newStr in ([], ['+'], ['-']):
            return 0
        elif -2147483648 <= int(''.join(newStr)) <= 2147483647:
            return int(''.join(newStr))
        elif int(''.join(newStr)) > 2147483647:
            return 2147483647
        else:
            return -2147483648

Without cheating:

class Solution:
    # @return an integer
    def atoi(self, Str):
        dic={"0":0,'1':1,'2':2,'3':3,'4':4,'5':5,'6':6,'7':7,'8':8,'9':9}
        rst=0
        validset='+-0123456789'
        numberset='0123456789'
        INT_MAX="2147483647"
        INT_MIN="2147483648"
        Str=Str.lstrip(' ')
        for i in range(len(Str)):
            if Str[i] not in validset:
                Str=Str[:i]
                break
        if (len(Str)==1 and Str in '+-') or ('-' in Str[1:] or '+' in Str[1:]) or len(Str)==0:
            return 0
        sign=1
        if Str[0]=='-':
            sign=-1
            Str=Str[1:]
        elif Str[0]=='+':
            Str=Str[1:]
        if len(Str)>10:
            return -2147483648 if sign==-1 else 2147483647
        elif len(Str)==10:
            overflow=True
            if sign==-1:
                for i in range(10):
                    if Str[i]<INT_MIN[i]:
                        overflow=False
                        break
                if overflow: 
                    return -2147483648
            else:
                for i in range(10):
                    if Str[i]<INT_MAX[i]:
                        overflow=False
                        break
                if overflow:
                    return 2147483647
            for i in range(len(Str)):
                if sign==1:
                    rst+=dic[Str[i]]*(10**(len(Str)-i-1))
                else:
                    rst-=dic[Str[i]]*(10**(len(Str)-i-1))
            return rst
        else:
            for i in range(len(Str)):
                rst+=dic[Str[i]]*(10**(len(Str)-i-1))
            return rst*sign