Is Python string immutable?

[could ildg]

In java and C# String is immutable, str=str+"some more" will return a new
string and leave some gargabe.
so in java and C# if there are some frequent string operation,
StringBuilder/StringBuffer is recommanded.

Will string operation in python also leave some garbage? I implemented a
net-spider in python which includes many html string procession. After it
running for sometime, the python exe eats up over 300M memory. Is this
because the string garbages?

If String in python is immutable, what class should I use to avoid too much
garbages when processing strings frequently?
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