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    <p>Some answers contain the word "copy" in a context of a function call. I find it confusing.</p> <p><strong>Python doesn't copy <em>objects</em> you pass during a function call <em>ever</em>.</strong></p> <p>Function parameters are <em>names</em>. When you call a function Python binds these parameters to whatever objects you pass (via names in a caller scope).</p> <p>Objects can be mutable (like lists) or immutable (like integers, strings in Python). Mutable object you can change. You can't change a name, you just can bind it to another object.</p> <p>Your example is not about <a href="https://docs.python.org/2/tutorial/classes.html#python-scopes-and-namespaces" rel="noreferrer">scopes or namespaces</a>, it is about <a href="http://docs.python.org/reference/executionmodel.html#naming-and-binding" rel="noreferrer">naming and binding</a> and <a href="http://docs.python.org/reference/datamodel.html#objects-values-and-types" rel="noreferrer">mutability of an object</a> in Python. </p> <pre><code>def f(n, x): # these `n`, `x` have nothing to do with `n` and `x` from main() n = 2 # put `n` label on `2` balloon x.append(4) # call `append` method of whatever object `x` is referring to. print 'In f():', n, x x = [] # put `x` label on `[]` ballon # x = [] has no effect on the original list that is passed into the function </code></pre> <p>Here are nice pictures on <a href="https://web.archive.org/web/20180121150727/http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables" rel="noreferrer">the difference between variables in other languages and names in Python</a>.</p>
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    1. POWhy can a function modify some arguments as perceived by the caller, but not others?
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    1. COThis article helped me to understand the problem better and it suggests a workaround and some advanced uses: [Default Parameter Values in Python](http://effbot.org/zone/default-values.htm)
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      1. USGfy
    2. CO@Gfy, I've seen similar examples before but to me it doesn't describe a real-world situation. If you're modifying something that's passed in it doesn't make sense to give it a default.
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      1. USMark Ransom
    3. CO@MarkRansom, I think it does make sense if you want to provide optional output destination as in: `def foo(x, l=None): l=l or []; l.append(x**2); return l[-1]`.
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      1. USJanusz Lenar
 

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