Why is the result of checks different?

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def checkio(data: str) -> bool:
    x='qwertyuioplkjhgfdsazxcvbnm'
    y='QWERTYUIOPLKJHGFDSAZXCVBNM'
    z='1234567890'
    b=data
    x1=0
    z1=0
    y1=0
    if len(b)<10 : return('False')
    else:
        for i in range(0,len(b)):
            if x.find(b[i])!=-1: x1=1+x1
            if z.find(b[i])!=-1: z1=1+z1
            if y.find(b[i])!=-1: y1=1+y1
        if x1!=0 and y1!=0 and z1!=0 :
            return ('True')
        else:
            return ('False')