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    <p>The short version is: Assignment doesn't mutate anything in Python, it just rebinds names. Rebinding a different name, that happens to be currently bound to a reference to the same string as <code>sys.ps1</code> doesn't affect <code>sys.ps1</code> in any way.</p> <p>Let's go through step by step:</p> <pre><code>from sys import ps1 </code></pre> <p>This imports <code>sys</code> (but doesn't bind a name <code>sys</code> in the current globals), then binds the name <code>ps1</code> in the current globals to the same object as <code>sys</code>'s <code>ps1</code>.</p> <pre><code>ps1 = 'something else ' </code></pre> <p>This rebinds the name <code>ps1</code> in the current globals to the same object as the literal <code>'something else'</code>. There's no way that could possibly affect the <code>sys</code> module.</p> <pre><code>import sys </code></pre> <p>This imports <code>sys</code>, and then binds the name <code>sys</code> in the current globals to it.</p> <pre><code>sys.ps1 = 'something else ' </code></pre> <p>This looks up the name <code>sys</code> in the current globals, getting a reference to the <code>sys</code> module, then rebinds the name <code>ps1</code> in that module to the same object as the literal <code>'something else '</code>.</p> <hr> <p>Or, putting it in pseudo-Python terms, instead of in English…</p> <p>Your first code sample is like this:</p> <pre><code>tempspace._sys = __import__('sys') globals.ps1 = tempspace._sys.ps1 del tempspace._sys globals.ps1 = 'something else ' </code></pre> <p>Your second is like this:</p> <pre><code>globals.sys = sys_modules.sys = __import__('sys') globals.sys.ps1 = 'something else ' </code></pre> <hr> <p>Here is a transcript of what you described in the comments.</p> <pre><code>Python 2.7.2 (default, Jun 20 2012, 16:23:33) [GCC 4.2.1 Compatible Apple Clang 4.0 (tags/Apple/clang-418.0.60)] on darwin Type "help", "copyright", "credits" or "license" for more information. &gt;&gt;&gt; import sys &gt;&gt;&gt; from sys import ps1 &gt;&gt;&gt; id(sys.ps1) 4360831168 &gt;&gt;&gt; id(ps1) 4360831168 &gt;&gt;&gt; sys.ps1 = 'something else ' something else ps1 '&gt;&gt;&gt; ' something else </code></pre> <p><code>sys.ps1</code> and <code>ps1</code> have the same ID, because they are two different names that reference the same object.</p> <p>And changing <code>sys.ps1</code> does not change <code>ps1</code>.</p>
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    1. POPython import and module scoping (specifically sys.ps1)
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    1. COIf it was binding ps1 to a copy of sys.ps1, wouldn't they have different IDs? Also, once I change sys.ps1, ps1 also changes.
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      1. USvictor
    2. CO@victor: Yes, if you bind to a copy they have different IDs, but if you bind to the same object, it's the same object, so of course it has the same ID. And when you change `sys.ps1`, `ps1` does _not_ change. Let me edit the answer to show you.
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    3. COAh hah. I guess I was actually confused about the flip side, which is also interesting. If you replace `sys.ps1 = 'something else '` with `ps1 = 'something else '`, then `ps1` changes its ID.
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