doubling a string by using 'for' loop

[python101]

I want to double a string by using 'for' loop in a special way for example:

 
string1 = 'abc'
#after doubled
string1 = 'aabbcc'
 

anyway, in python what does "+=" mean?

[ghostdog74]

I want to double a string by using 'for' loop in a special way for example:

 
string1 = 'abc'
#after doubled
string1 = 'aabbcc'
 

anyway, in python what does "+=" mean?

>>> s="abc"
>>> ''.join([i*2 for i in s ])
'aabbcc'
 

c+=1 means the same as c=c+1