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<li><a class="reference internal" href="#">uuid – Universally unique identifiers</a><ul>
<li><a class="reference internal" href="#uuid-1-ieee-802-mac-address">UUID 1 - IEEE 802 MAC Address</a></li>
<li><a class="reference internal" href="#uuid-3-and-5-name-based-values">UUID 3 and 5 - Name-Based Values</a></li>
<li><a class="reference internal" href="#uuid-4-random-values">UUID 4 - Random Values</a></li>
<li><a class="reference internal" href="#working-with-uuid-objects">Working with UUID Objects</a></li>
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<div class="section" id="module-uuid">
<span id="uuid-universally-unique-identifiers"></span><h1>uuid – Universally unique identifiers<a class="headerlink" href="#module-uuid" title="Permalink to this headline">¶</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Purpose:</th><td class="field-body">The <a class="reference internal" href="#module-uuid" title="uuid: Universally unique identifiers"><tt class="xref py py-mod docutils literal"><span class="pre">uuid</span></tt></a> module implements Universally Unique Identifiers as described in <span class="target" id="index-0"></span><a class="rfc reference external" href="http://tools.ietf.org/html/rfc4122.html"><strong>RFC 4122</strong></a>.</td>
</tr>
<tr class="field"><th class="field-name">Python Version:</th><td class="field-body">2.5 and later</td>
</tr>
</tbody>
</table>
<p><span class="target" id="index-1"></span><a class="rfc reference external" href="http://tools.ietf.org/html/rfc4122.html"><strong>RFC 4122</strong></a> defines a system for creating universally unique
identifiers for resources in a way that does not require a central
registrar. UUID values are 128 bits long and “can guarantee uniqueness
across space and time”. They are useful for identifiers for documents,
hosts, application clients, and other situations where a unique value
is necessary. The RFC is specifically geared toward creating a Uniform
Resource Name namespace.</p>
<p>Three main algorithms are covered by the spec:</p>
<ul class="simple">
<li>Using IEEE 802 MAC addresses as a source of uniqueness</li>
<li>Using pseudo-random numbers</li>
<li>Using well-known strings combined with cryptographic hashing</li>
</ul>
<p>In all cases the seed value is combined with the system clock and a
clock sequence value (to maintain uniqueness in case the clock was set
backwards).</p>
<div class="section" id="uuid-1-ieee-802-mac-address">
<h2>UUID 1 - IEEE 802 MAC Address<a class="headerlink" href="#uuid-1-ieee-802-mac-address" title="Permalink to this headline">¶</a></h2>
<p>UUID version 1 values are computed using the MAC address of the host.
The <a class="reference internal" href="#module-uuid" title="uuid: Universally unique identifiers"><tt class="xref py py-mod docutils literal"><span class="pre">uuid</span></tt></a> module uses <tt class="xref py py-func docutils literal"><span class="pre">getnode()</span></tt> to retrieve the MAC
value on a given system:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">uuid</span>
<span class="k">print</span> <span class="nb">hex</span><span class="p">(</span><span class="n">uuid</span><span class="o">.</span><span class="n">getnode</span><span class="p">())</span>
</pre></div>
</div>
<div class="highlight-python"><pre>$ python uuid_getnode.py
0x1ec200d9e0</pre>
</div>
<p>If a system has more than one network card, and so more than one MAC,
any one of the values may be returned.</p>
<p>To generate a UUID for a given host, identified by its MAC address,
use the <tt class="xref py py-func docutils literal"><span class="pre">uuid1()</span></tt> function. You can pass a node identifier, or
leave the field blank to use the value returned by <tt class="xref py py-func docutils literal"><span class="pre">getnode()</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">uuid</span>
<span class="n">u</span> <span class="o">=</span> <span class="n">uuid</span><span class="o">.</span><span class="n">uuid1</span><span class="p">()</span>
<span class="k">print</span> <span class="n">u</span>
<span class="k">print</span> <span class="nb">type</span><span class="p">(</span><span class="n">u</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'bytes :'</span><span class="p">,</span> <span class="nb">repr</span><span class="p">(</span><span class="n">u</span><span class="o">.</span><span class="n">bytes</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'hex :'</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">hex</span>
<span class="k">print</span> <span class="s">'int :'</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">int</span>
<span class="k">print</span> <span class="s">'urn :'</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">urn</span>
<span class="k">print</span> <span class="s">'variant :'</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">variant</span>
<span class="k">print</span> <span class="s">'version :'</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">version</span>
<span class="k">print</span> <span class="s">'fields :'</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">fields</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">time_low : '</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">time_low</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">time_mid : '</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">time_mid</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">time_hi_version : '</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">time_hi_version</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">clock_seq_hi_variant: '</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">clock_seq_hi_variant</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">clock_seq_low : '</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">clock_seq_low</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">node : '</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">node</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">time : '</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">time</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">clock_seq : '</span><span class="p">,</span> <span class="n">u</span><span class="o">.</span><span class="n">clock_seq</span>
</pre></div>
</div>
<p>The components of the UUID object returned can be accessed through
read-only instance attributes. Some attributes, such as <em>hex</em>, <em>int</em>,
and <em>urn</em>, are different representations of the UUID value.</p>
<div class="highlight-python"><pre>$ python uuid_uuid1.py
098c95b3-df6e-11df-899a-001ec200d9e0
<class 'uuid.UUID'>
bytes : '\t\x8c\x95\xb3\xdfn\x11\xdf\x89\x9a\x00\x1e\xc2\x00\xd9\xe0'
hex : 098c95b3df6e11df899a001ec200d9e0
int : 12693009852279376310425064831178562016
urn : urn:uuid:098c95b3-df6e-11df-899a-001ec200d9e0
variant : specified in RFC 4122
version : 1
fields : (160208307L, 57198L, 4575L, 137L, 154L, 132103854560L)
time_low : 160208307
time_mid : 57198
time_hi_version : 4575
clock_seq_hi_variant: 137
clock_seq_low : 154
node : 132103854560
time : 135072177544009139
clock_seq : 2458</pre>
</div>
<p>Because of the time component, each time <tt class="xref py py-func docutils literal"><span class="pre">uuid1()</span></tt> is called a
new value is returned.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">uuid</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">xrange</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span>
<span class="k">print</span> <span class="n">uuid</span><span class="o">.</span><span class="n">uuid1</span><span class="p">()</span>
</pre></div>
</div>
<p>Notice in this output that only the time component (at the beginning
of the string) changes.</p>
<div class="highlight-python"><pre>$ python uuid_uuid1_repeat.py
099b0e00-df6e-11df-9acd-001ec200d9e0
099cea35-df6e-11df-8121-001ec200d9e0
099cebcf-df6e-11df-bc49-001ec200d9e0</pre>
</div>
<p>Because your computer has a different MAC address than mine, you will
see entirely different values if you run the examples, because the
node identifier at the end of the UUID will change, too.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">uuid</span>
<span class="n">node1</span> <span class="o">=</span> <span class="n">uuid</span><span class="o">.</span><span class="n">getnode</span><span class="p">()</span>
<span class="k">print</span> <span class="nb">hex</span><span class="p">(</span><span class="n">node1</span><span class="p">),</span> <span class="n">uuid</span><span class="o">.</span><span class="n">uuid1</span><span class="p">(</span><span class="n">node1</span><span class="p">)</span>
<span class="n">node2</span> <span class="o">=</span> <span class="mh">0x1e5274040e</span>
<span class="k">print</span> <span class="nb">hex</span><span class="p">(</span><span class="n">node2</span><span class="p">),</span> <span class="n">uuid</span><span class="o">.</span><span class="n">uuid1</span><span class="p">(</span><span class="n">node2</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-python"><pre>$ python uuid_uuid1_othermac.py
0x1ec200d9e0 09af7551-df6e-11df-b2e2-001ec200d9e0
0x1e5274040e 09b00e5e-df6e-11df-a0c4-001e5274040e</pre>
</div>
</div>
<div class="section" id="uuid-3-and-5-name-based-values">
<h2>UUID 3 and 5 - Name-Based Values<a class="headerlink" href="#uuid-3-and-5-name-based-values" title="Permalink to this headline">¶</a></h2>
<p>It is also useful in some contexts to create UUID values from names
instead of random or time-based values. Versions 3 and 5 of the UUID
specification use cryptographic hash values (MD5 or SHA-1) to combine
namespace-specific seed values with “names” (DNS hostnames, URLs,
object ids, etc.). There are several well-known namespaces, identified
by pre-defined UUID values, for working with DNS, URLs, ISO OIDs, and
X.500 Distinguished Names. You can also define your own application-
specific namespaces by generating and saving UUID values.</p>
<p>To create a UUID from a DNS name, pass <tt class="docutils literal"><span class="pre">uuid.NAMESPACE_DNS</span></tt> as the
namespace argument to <tt class="xref py py-func docutils literal"><span class="pre">uuid3()</span></tt> or <tt class="xref py py-func docutils literal"><span class="pre">uuid5()</span></tt>:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">uuid</span>
<span class="n">hostnames</span> <span class="o">=</span> <span class="p">[</span><span class="s">'www.doughellmann.com'</span><span class="p">,</span> <span class="s">'blog.doughellmann.com'</span><span class="p">]</span>
<span class="k">for</span> <span class="n">name</span> <span class="ow">in</span> <span class="n">hostnames</span><span class="p">:</span>
<span class="k">print</span> <span class="n">name</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">MD5 :'</span><span class="p">,</span> <span class="n">uuid</span><span class="o">.</span><span class="n">uuid3</span><span class="p">(</span><span class="n">uuid</span><span class="o">.</span><span class="n">NAMESPACE_DNS</span><span class="p">,</span> <span class="n">name</span><span class="p">)</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">SHA-1 :'</span><span class="p">,</span> <span class="n">uuid</span><span class="o">.</span><span class="n">uuid5</span><span class="p">(</span><span class="n">uuid</span><span class="o">.</span><span class="n">NAMESPACE_DNS</span><span class="p">,</span> <span class="n">name</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-python"><pre>$ python uuid_uuid3_uuid5.py
www.doughellmann.com
MD5 : bcd02e22-68f0-3046-a512-327cca9def8f
SHA-1 : e3329b12-30b7-57c4-8117-c2cd34a87ce9
blog.doughellmann.com
MD5 : 9bdabfce-dfd6-37ab-8a3f-7f7293bcf111
SHA-1 : fa829736-7ef8-5239-9906-b4775a5abacb</pre>
</div>
<p>The UUID value for a given name in a namespace is always the same, no
matter when or where it is calculated. Values for the same name in
different namespaces are different.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">uuid</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">xrange</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span>
<span class="k">print</span> <span class="n">uuid</span><span class="o">.</span><span class="n">uuid3</span><span class="p">(</span><span class="n">uuid</span><span class="o">.</span><span class="n">NAMESPACE_DNS</span><span class="p">,</span> <span class="s">'www.doughellmann.com'</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-python"><pre>$ python uuid_uuid3_repeat.py
bcd02e22-68f0-3046-a512-327cca9def8f
bcd02e22-68f0-3046-a512-327cca9def8f
bcd02e22-68f0-3046-a512-327cca9def8f</pre>
</div>
</div>
<div class="section" id="uuid-4-random-values">
<h2>UUID 4 - Random Values<a class="headerlink" href="#uuid-4-random-values" title="Permalink to this headline">¶</a></h2>
<p>Sometimes host-based and namespace-based UUID values are not
“different enough”. For example, in cases where you want to use the
UUID as a lookup key, a more random sequence of values with more
differentiation is desirable to avoid collisions in a hash
table. Having values with fewer common digits also makes it easier to
find them in log files. To add greater differentiation in your UUIDs,
use <tt class="xref py py-func docutils literal"><span class="pre">uuid4()</span></tt> to generate them using random input values.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">uuid</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">xrange</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span>
<span class="k">print</span> <span class="n">uuid</span><span class="o">.</span><span class="n">uuid4</span><span class="p">()</span>
</pre></div>
</div>
<div class="highlight-python"><pre>$ python uuid_uuid4.py
05082f2e-6e12-4016-8b07-7970f9dcf9a7
60aae332-5119-40d0-9bb7-174e53454654
671bdeb2-fbee-458c-9b6f-55764054e635</pre>
</div>
</div>
<div class="section" id="working-with-uuid-objects">
<h2>Working with UUID Objects<a class="headerlink" href="#working-with-uuid-objects" title="Permalink to this headline">¶</a></h2>
<p>In addition to generating new UUID values, you can parse strings in
various formats to create UUID objects. This makes it easier to
compare them, sort them, etc.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">uuid</span>
<span class="k">def</span> <span class="nf">show</span><span class="p">(</span><span class="n">msg</span><span class="p">,</span> <span class="n">l</span><span class="p">):</span>
<span class="k">print</span> <span class="n">msg</span>
<span class="k">for</span> <span class="n">v</span> <span class="ow">in</span> <span class="n">l</span><span class="p">:</span>
<span class="k">print</span> <span class="s">'</span><span class="se">\t</span><span class="s">'</span><span class="p">,</span> <span class="n">v</span>
<span class="k">print</span>
<span class="n">input_values</span> <span class="o">=</span> <span class="p">[</span>
<span class="s">'urn:uuid:f2f84497-b3bf-493a-bba9-7c68e6def80b'</span><span class="p">,</span>
<span class="s">'{417a5ebb-01f7-4ed5-aeac-3d56cd5037b0}'</span><span class="p">,</span>
<span class="s">'2115773a-5bf1-11dd-ab48-001ec200d9e0'</span><span class="p">,</span>
<span class="p">]</span>
<span class="n">show</span><span class="p">(</span><span class="s">'input_values'</span><span class="p">,</span> <span class="n">input_values</span><span class="p">)</span>
<span class="n">uuids</span> <span class="o">=</span> <span class="p">[</span> <span class="n">uuid</span><span class="o">.</span><span class="n">UUID</span><span class="p">(</span><span class="n">s</span><span class="p">)</span> <span class="k">for</span> <span class="n">s</span> <span class="ow">in</span> <span class="n">input_values</span> <span class="p">]</span>
<span class="n">show</span><span class="p">(</span><span class="s">'converted to uuids'</span><span class="p">,</span> <span class="n">uuids</span><span class="p">)</span>
<span class="n">uuids</span><span class="o">.</span><span class="n">sort</span><span class="p">()</span>
<span class="n">show</span><span class="p">(</span><span class="s">'sorted'</span><span class="p">,</span> <span class="n">uuids</span><span class="p">)</span>
</pre></div>
</div>
<div class="highlight-python"><pre>$ python uuid_uuid_objects.py
input_values
urn:uuid:f2f84497-b3bf-493a-bba9-7c68e6def80b
{417a5ebb-01f7-4ed5-aeac-3d56cd5037b0}
2115773a-5bf1-11dd-ab48-001ec200d9e0
converted to uuids
f2f84497-b3bf-493a-bba9-7c68e6def80b
417a5ebb-01f7-4ed5-aeac-3d56cd5037b0
2115773a-5bf1-11dd-ab48-001ec200d9e0
sorted
2115773a-5bf1-11dd-ab48-001ec200d9e0
417a5ebb-01f7-4ed5-aeac-3d56cd5037b0
f2f84497-b3bf-493a-bba9-7c68e6def80b</pre>
</div>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><a class="reference external" href="http://docs.python.org/lib/module-uuid.html">uuid</a></dt>
<dd>Standard library documentation for this module.</dd>
<dt><span class="target" id="index-2"></span><a class="rfc reference external" href="http://tools.ietf.org/html/rfc4122.html"><strong>RFC 4122</strong></a></dt>
<dd>A Universally Unique IDentifier (UUID) URN Namespace</dd>
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