Why hasn't this list changed? : <p>In the following code, the function modify_list want to modify b, but it failed, the print result is [1,2,3]. why hasn't the list a changed?</p>

<pre><code>def modify_list(b):
    b = [1,2]
a = [1,2,3]
modify_list(a)
print(a)
</code></pre>
