String assignment
<p>I have a look at python code where string variable assignments looks like:</p>

<pre><code>var1 = var2[:]
</code></pre>

<p>I am just wondering what is the difference between:</p>

<pre><code>var1 = var2
</code></pre>

<p>Here is my experiments:</p>

<pre><code>&gt;&gt;&gt; original = "some text"
&gt;&gt;&gt; copy1 = original
&gt;&gt;&gt; copy2 = original[:]
&gt;&gt;&gt; original = "another text"
&gt;&gt;&gt; copy1
'some text'
&gt;&gt;&gt; copy2
'some text'
</code></pre>

<p><strong>Updated:</strong></p>

<p>Here is a full <a href="http://pastebin.com/CvTgkPz9" rel="nofollow">code</a>.  This code search a key for substitution cipher.
If I remove '[:]' this code will works very slowly.</p>
