If condition is getting skipped even if the condition is true

I am using an If statement inside a for loop but the If statement is getting skipped even after the condition is met

x=raw_input().split(" ")  
c=[]  
for a in x:  
    b=1  
    if a<0:  
        print "Please enter a number greater than or equal to 0"  
    else:      
        if(a==1 or a==0 ):  
            print "1"  
    for i in range(1,int(a)+1):  
            b=b*i  
    c.append(str(b))  

print ",".join(c)

the program is to find factorial, i am getting the result. If someone enters a negative number, it should not return a factorial but this does. I just want to know why is the if and else conditions getting skipped.