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@ -21,7 +21,7 @@ A collection of useful scripts, tutorials, and other Python-related things
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- Python's scope resolution for variable names and the LEGB rule [[IPython nb](http://nbviewer.ipython.org/github/rasbt/python_reference/blob/master/tutorials/scope_resolution_legb_rule.ipynb?create=1)]
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- Key differences between Python 2.x and Python 3.x [[Markdown](./tutorials/key_differences_python2_python3.md)]
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- Key differences between Python 2.x and Python 3.x [[HTML](http://htmlpreview.github.io/?https://github.com/rasbt/python_reference/blob/master/tutorials/key_differences_between_python_2_and_3.html)]
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- A thorough guide to SQLite database operations in Python [[Markdown](./sqlite3_howto/README.md)]
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tutorials/key_differences_between_python_2_and_3.html
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tutorials/key_differences_between_python_2_and_3.html
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</style>
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<title>Key differences between Python 2 and 3</title>
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</head>
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<body>
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<p><a href="http://sebastianraschka.com">Sebastian Raschka</a><br/>
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last updated: 05/24/2014</p>
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<p><br></p>
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<p><strong>This is a subsection of <a href="http://nbviewer.ipython.org/github/rasbt/python_reference/blob/master/not_so_obvious_python_stuff.ipynb?create=1">"A collection of not-so-obvious Python stuff you should know!"</a></strong></p>
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<p><a id='sections'></a></p>
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<p><br></p>
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<h2>Key differences between Python 2 and 3</h2>
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<p><br></p>
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<p>There are some good articles already that are summarizing the differences between Python 2 and 3, e.g.,<br/>
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- <a href="https://wiki.python.org/moin/Python2orPython3">https://wiki.python.org/moin/Python2orPython3</a>
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- <a href="https://docs.python.org/3.0/whatsnew/3.0.html">https://docs.python.org/3.0/whatsnew/3.0.html</a>
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- <a href="http://python3porting.com/differences.html">http://python3porting.com/differences.html</a>
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- <a href="https://docs.python.org/3/howto/pyporting.html">https://docs.python.org/3/howto/pyporting.html</a><br/>
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etc.</p>
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<p>But it might be still worthwhile, especially for Python newcomers, to take a look at some of those!
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(Note: the the code was executed in Python 3.4.0 and Python 2.7.5 and copied from interactive shell sessions.)</p>
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<p><a id='py23_overview'></a></p>
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<p><br></p>
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<h3>Overview - Key differences between Python 2 and 3</h3>
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<ul>
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<li><a href="#unicode">Unicode</a></li>
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<li><a href="#print">The print statement</a></li>
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<li><a href="#integer_div">Integer division</a></li>
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<li><a href="#xrange">xrange()</a></li>
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<li><a href="#raising_exceptions">Raising exceptions</a></li>
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<li><a href="#handling_exceptions">Handling exceptions</a></li>
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<li><a href="#next_next">next() function and .next() method</a></li>
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<li><a href="#loop_leak">Loop variables and leaking into the global scope</a></li>
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<li><a href="#compare_unorder">Comparing unorderable types</a></li>
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</ul>
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<p><br>
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<br></p>
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<p><a id='unicode'></a>
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<br>
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<br></p>
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<h3>Unicode...</h3>
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<p>[<a href="#py23_overview">back to Python 2.x vs 3.x overview</a>]</p>
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<h4>- Python 2:</h4>
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<p>We have ASCII <code>str()</code> types, separate <code>unicode()</code>, but no <code>byte</code> type</p>
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<h4>- Python 3:</h4>
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<p>Now, we finally have Unicode (utf-8) <code>str</code>ings, and 2 byte classes: <code>byte</code> and <code>bytearray</code>s</p>
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<p><br></p>
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<!-- HTML generated using hilite.me -->
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<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%"><span style="color: #888888">#############</span>
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<span style="color: #888888"># Python 2</span>
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<span style="color: #888888">#############</span>
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<span style="color: #333333">>>></span> <span style="color: #007020">type</span>(unicode(<span style="background-color: #fff0f0">'is like a python3 str()'</span>))
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<span style="color: #333333"><</span><span style="color: #007020">type</span> <span style="background-color: #fff0f0">'unicode'</span><span style="color: #333333">></span>
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<span style="color: #333333">>>></span> <span style="color: #007020">type</span>(b<span style="background-color: #fff0f0">'byte type does not exist'</span>)
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<span style="color: #333333"><</span><span style="color: #007020">type</span> <span style="background-color: #fff0f0">'str'</span><span style="color: #333333">></span>
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<span style="color: #333333">>>></span> <span style="background-color: #fff0f0">'they are really'</span> <span style="color: #333333">+</span> b<span style="background-color: #fff0f0">' the same'</span>
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<span style="background-color: #fff0f0">'they are really the same'</span>
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<span style="color: #333333">>>></span> <span style="color: #007020">type</span>(<span style="color: #007020">bytearray</span>(b<span style="background-color: #fff0f0">'bytearray oddly does exist though'</span>))
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<span style="color: #333333"><</span><span style="color: #007020">type</span> <span style="background-color: #fff0f0">'bytearray'</span><span style="color: #333333">></span>
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<span style="color: #888888">#############</span>
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<span style="color: #888888"># Python 3</span>
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<span style="color: #888888">#############</span>
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<span style="color: #333333">>>></span> <span style="color: #007020">print</span>(<span style="background-color: #fff0f0">'strings are now utf-8 </span><span style="color: #666666; font-weight: bold; background-color: #fff0f0">\u03BC</span><span style="background-color: #fff0f0">nico</span><span style="color: #666666; font-weight: bold; background-color: #fff0f0">\u0394</span><span style="background-color: #fff0f0">é!'</span>)
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strings are now utf<span style="color: #333333">-</span><span style="color: #0000DD; font-weight: bold">8</span> μnicoΔé<span style="color: #FF0000; background-color: #FFAAAA">!</span>
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<span style="color: #333333">>>></span> <span style="color: #007020">type</span>(b<span style="background-color: #fff0f0">' and we have byte types for storing data'</span>)
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<span style="color: #333333"><</span><span style="color: #008800; font-weight: bold">class</span> <span style="color: #FF0000; background-color: #FFAAAA">'</span><span style="color: #BB0066; font-weight: bold">bytes</span><span style="background-color: #fff0f0">'></span>
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<span style="color: #333333">>>></span> <span style="color: #007020">type</span>(<span style="color: #007020">bytearray</span>(b<span style="background-color: #fff0f0">'but also bytearrays for those who prefer them over strings'</span>))
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<span style="color: #333333"><</span><span style="color: #008800; font-weight: bold">class</span> <span style="color: #FF0000; background-color: #FFAAAA">'</span><span style="color: #BB0066; font-weight: bold">bytearray</span><span style="background-color: #fff0f0">'></span>
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<span style="color: #333333">>>></span> <span style="background-color: #fff0f0">'string'</span> <span style="color: #333333">+</span> b<span style="background-color: #fff0f0">'bytes for data'</span>
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Traceback (most recent call last):s
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File <span style="background-color: #fff0f0">"<stdin>"</span>, line <span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #000000; font-weight: bold">in</span> <span style="color: #333333"><</span>module<span style="color: #333333">></span>
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<span style="color: #FF0000; font-weight: bold">TypeError</span>: Can<span style="background-color: #fff0f0">'t convert '</span><span style="color: #007020">bytes</span><span style="background-color: #fff0f0">' object to str implicitly</span>
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</pre></div>
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<p><a id='print'></a>
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<br>
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<br></p>
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<h3>The print statement</h3>
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<p>[<a href="#py23_overview">back to Python 2.x vs 3.x overview</a>]</p>
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<p>Very trivial, but this change makes sense, Python 3 now only accepts <code>print</code>s with proper parentheses - just like the other function calls ...</p>
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<p><br></p>
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<!-- HTML generated using hilite.me -->
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<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%"><span style="color: #888888"># Python 2</span>
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<span style="color: #333333">>>></span> <span style="color: #007020">print</span> <span style="background-color: #fff0f0">'Hello, World!'</span>
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Hello, World<span style="color: #FF0000; background-color: #FFAAAA">!</span>
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<span style="color: #333333">>>></span> <span style="color: #007020">print</span>(<span style="background-color: #fff0f0">'Hello, World!'</span>)
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Hello, World<span style="color: #FF0000; background-color: #FFAAAA">!</span>
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<span style="color: #888888"># Python 3</span>
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<span style="color: #333333">>>></span> <span style="color: #007020">print</span>(<span style="background-color: #fff0f0">'Hello, World!'</span>)
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Hello, World<span style="color: #FF0000; background-color: #FFAAAA">!</span>
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<span style="color: #333333">>>></span> <span style="color: #007020">print</span> <span style="background-color: #fff0f0">'Hello, World!'</span>
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File <span style="background-color: #fff0f0">"<stdin>"</span>, line <span style="color: #0000DD; font-weight: bold">1</span>
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<span style="color: #007020">print</span> <span style="background-color: #fff0f0">'Hello, World!'</span>
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<span style="color: #333333">^</span>
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<span style="color: #FF0000; font-weight: bold">SyntaxError</span>: invalid syntax
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</pre></div>
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<p><br></p>
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<p>And if we want to print the output of 2 consecutive print functions on the same line, you would use a comma in Python 2, and a <code>end=""</code> in Python 3:</p>
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<p><br></p>
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<!-- HTML generated using hilite.me -->
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<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%"><span style="color: #888888"># Python 2</span>
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<span style="color: #333333">>>></span> <span style="color: #007020">print</span> <span style="background-color: #fff0f0">"line 1"</span>, ; <span style="color: #007020">print</span> <span style="background-color: #fff0f0">'same line'</span>
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line <span style="color: #0000DD; font-weight: bold">1</span> same line
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<span style="color: #888888"># Python 3</span>
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<span style="color: #333333">>>></span> <span style="color: #007020">print</span>(<span style="background-color: #fff0f0">"line 1"</span>, end<span style="color: #333333">=</span><span style="background-color: #fff0f0">""</span>) ; <span style="color: #007020">print</span> (<span style="background-color: #fff0f0">" same line"</span>)
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line <span style="color: #0000DD; font-weight: bold">1</span> same line
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</pre></div>
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<p><a id='integer_div'></a>
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<br>
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<br></p>
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<h3>Integer division</h3>
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<p>[<a href="#py23_overview">back to Python 2.x vs 3.x overview</a>]</p>
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<p>This is a pretty dangerous thing if you are porting code, or executing Python 3 code in Python 2 since the change in integer-division behavior can often go unnoticed.<br/>
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So, I still tend to use a <code>float(3)/2</code> or <code>3/2.0</code> instead of a <code>3/2</code> in my Python 3 scripts to save the Python 2 guys some trouble ... (PS: and vice versa, you can <code>from __future__ import division</code> in your Python 2 scripts).</p>
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<p><br></p>
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<!-- HTML generated using hilite.me -->
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<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%"><span style="color: #888888"># Python 2</span>
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<span style="color: #333333">>>></span> <span style="color: #0000DD; font-weight: bold">3</span> <span style="color: #333333">/</span> <span style="color: #0000DD; font-weight: bold">2</span>
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<span style="color: #0000DD; font-weight: bold">1</span>
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<span style="color: #333333">>>></span> <span style="color: #0000DD; font-weight: bold">3</span> <span style="color: #333333">//</span> <span style="color: #0000DD; font-weight: bold">2</span>
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<span style="color: #0000DD; font-weight: bold">1</span>
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<span style="color: #333333">>>></span> <span style="color: #0000DD; font-weight: bold">3</span> <span style="color: #333333">/</span> <span style="color: #6600EE; font-weight: bold">2.0</span>
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<span style="color: #6600EE; font-weight: bold">1.5</span>
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<span style="color: #333333">>>></span> <span style="color: #0000DD; font-weight: bold">3</span> <span style="color: #333333">//</span> <span style="color: #6600EE; font-weight: bold">2.0</span>
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<span style="color: #6600EE; font-weight: bold">1.0</span>
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<span style="color: #888888"># Python 3</span>
|
||||
<span style="color: #333333">>>></span> <span style="color: #0000DD; font-weight: bold">3</span> <span style="color: #333333">/</span> <span style="color: #0000DD; font-weight: bold">2</span>
|
||||
<span style="color: #6600EE; font-weight: bold">1.5</span>
|
||||
<span style="color: #333333">>>></span> <span style="color: #0000DD; font-weight: bold">3</span> <span style="color: #333333">//</span> <span style="color: #0000DD; font-weight: bold">2</span>
|
||||
<span style="color: #0000DD; font-weight: bold">1</span>
|
||||
<span style="color: #333333">>>></span> <span style="color: #0000DD; font-weight: bold">3</span> <span style="color: #333333">/</span> <span style="color: #6600EE; font-weight: bold">2.0</span>
|
||||
<span style="color: #6600EE; font-weight: bold">1.5</span>
|
||||
<span style="color: #333333">>>></span> <span style="color: #0000DD; font-weight: bold">3</span> <span style="color: #333333">//</span> <span style="color: #6600EE; font-weight: bold">2.0</span>
|
||||
<span style="color: #6600EE; font-weight: bold">1.0</span>
|
||||
</pre></div>
|
||||
|
||||
|
||||
<p><a id='xrange'></a>
|
||||
<br>
|
||||
<br></p>
|
||||
|
||||
<h3><code>xrange()</code></h3>
|
||||
|
||||
<p>[<a href="#py23_overview">back to Python 2.x vs 3.x overview</a>]</p>
|
||||
|
||||
<p><code>xrange()</code> was pretty popular in Python 2.x if you wanted to create an iterable object. The behavior was quite similar to a generator ('lazy evaluation'), but you could iterate over it infinitely. The advantage was that it was generally faster than <code>range()</code> (e.g., in a for-loop) - not if you had to iterate over the list multiple times, since the generation happens every time from scratch!<br/>
|
||||
In Python 3, the <code>range()</code> was implemented like the <code>xrange()</code> function so that a dedicated <code>xrange()</code> function does not exist anymore.</p>
|
||||
|
||||
<!-- HTML generated using hilite.me -->
|
||||
|
||||
|
||||
<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%"><span style="color: #888888"># Python 2</span>
|
||||
<span style="color: #333333">></span> python <span style="color: #333333">-</span>m timeit <span style="background-color: #fff0f0">'for i in range(1000000):'</span> <span style="background-color: #fff0f0">' pass'</span>
|
||||
<span style="color: #0000DD; font-weight: bold">10</span> loops, best of <span style="color: #0000DD; font-weight: bold">3</span>: <span style="color: #0000DD; font-weight: bold">66</span> msec per loop
|
||||
|
||||
<span style="color: #333333">></span> python <span style="color: #333333">-</span>m timeit <span style="background-color: #fff0f0">'for i in xrange(1000000):'</span> <span style="background-color: #fff0f0">' pass'</span>
|
||||
<span style="color: #0000DD; font-weight: bold">10</span> loops, best of <span style="color: #0000DD; font-weight: bold">3</span>: <span style="color: #6600EE; font-weight: bold">27.8</span> msec per loop
|
||||
|
||||
<span style="color: #888888"># Python 3</span>
|
||||
<span style="color: #333333">></span> python3 <span style="color: #333333">-</span>m timeit <span style="background-color: #fff0f0">'for i in range(1000000):'</span> <span style="background-color: #fff0f0">' pass'</span>
|
||||
<span style="color: #0000DD; font-weight: bold">10</span> loops, best of <span style="color: #0000DD; font-weight: bold">3</span>: <span style="color: #6600EE; font-weight: bold">51.1</span> msec per loop
|
||||
|
||||
<span style="color: #333333">></span> python3 <span style="color: #333333">-</span>m timeit <span style="background-color: #fff0f0">'for i in xrange(1000000):'</span> <span style="background-color: #fff0f0">' pass'</span>
|
||||
Traceback (most recent call last):
|
||||
File <span style="background-color: #fff0f0">"/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/timeit.py"</span>, line <span style="color: #0000DD; font-weight: bold">292</span>, <span style="color: #000000; font-weight: bold">in</span> main
|
||||
x <span style="color: #333333">=</span> t<span style="color: #333333">.</span>timeit(number)
|
||||
File <span style="background-color: #fff0f0">"/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/timeit.py"</span>, line <span style="color: #0000DD; font-weight: bold">178</span>, <span style="color: #000000; font-weight: bold">in</span> timeit
|
||||
timing <span style="color: #333333">=</span> <span style="color: #007020">self</span><span style="color: #333333">.</span>inner(it, <span style="color: #007020">self</span><span style="color: #333333">.</span>timer)
|
||||
File <span style="background-color: #fff0f0">"<timeit-src>"</span>, line <span style="color: #0000DD; font-weight: bold">6</span>, <span style="color: #000000; font-weight: bold">in</span> inner
|
||||
<span style="color: #008800; font-weight: bold">for</span> i <span style="color: #000000; font-weight: bold">in</span> xrange(<span style="color: #0000DD; font-weight: bold">1000000</span>):
|
||||
<span style="color: #FF0000; font-weight: bold">NameError</span>: name <span style="background-color: #fff0f0">'xrange'</span> <span style="color: #000000; font-weight: bold">is</span> <span style="color: #000000; font-weight: bold">not</span> defined
|
||||
</pre></div>
|
||||
|
||||
|
||||
<p><a id='raising_exceptions'></a>
|
||||
<br>
|
||||
<br></p>
|
||||
|
||||
<h3>Raising exceptions</h3>
|
||||
|
||||
<p>[<a href="#py23_overview">back to Python 2.x vs 3.x overview</a>]</p>
|
||||
|
||||
<p>Where Python 2 accepts both notations, the 'old' and the 'new' way, Python 3 chokes (and raises a <code>SyntaxError</code> in turn) if we don't enclose the exception argument in parentheses:</p>
|
||||
|
||||
<p><br></p>
|
||||
|
||||
<!-- HTML generated using hilite.me -->
|
||||
|
||||
|
||||
<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%"><span style="color: #888888"># Python 2</span>
|
||||
<span style="color: #333333">>>></span> <span style="color: #008800; font-weight: bold">raise</span> <span style="color: #FF0000; font-weight: bold">IOError</span>, <span style="background-color: #fff0f0">"file error"</span>
|
||||
Traceback (most recent call last):
|
||||
File <span style="background-color: #fff0f0">"<stdin>"</span>, line <span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #000000; font-weight: bold">in</span> <span style="color: #333333"><</span>module<span style="color: #333333">></span>
|
||||
<span style="color: #FF0000; font-weight: bold">IOError</span>: file error
|
||||
<span style="color: #333333">>>></span> <span style="color: #008800; font-weight: bold">raise</span> <span style="color: #FF0000; font-weight: bold">IOError</span>(<span style="background-color: #fff0f0">"file error"</span>)
|
||||
Traceback (most recent call last):
|
||||
File <span style="background-color: #fff0f0">"<stdin>"</span>, line <span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #000000; font-weight: bold">in</span> <span style="color: #333333"><</span>module<span style="color: #333333">></span>
|
||||
<span style="color: #FF0000; font-weight: bold">IOError</span>: file error
|
||||
|
||||
|
||||
<span style="color: #888888"># Python 3 </span>
|
||||
<span style="color: #333333">>>></span> <span style="color: #008800; font-weight: bold">raise</span> <span style="color: #FF0000; font-weight: bold">IOError</span>, <span style="background-color: #fff0f0">"file error"</span>
|
||||
File <span style="background-color: #fff0f0">"<stdin>"</span>, line <span style="color: #0000DD; font-weight: bold">1</span>
|
||||
<span style="color: #008800; font-weight: bold">raise</span> <span style="color: #FF0000; font-weight: bold">IOError</span>, <span style="background-color: #fff0f0">"file error"</span>
|
||||
<span style="color: #333333">^</span>
|
||||
<span style="color: #FF0000; font-weight: bold">SyntaxError</span>: invalid syntax
|
||||
<span style="color: #333333">>>></span> <span style="color: #008800; font-weight: bold">raise</span> <span style="color: #FF0000; font-weight: bold">IOError</span>(<span style="background-color: #fff0f0">"file error"</span>)
|
||||
Traceback (most recent call last):
|
||||
File <span style="background-color: #fff0f0">"<stdin>"</span>, line <span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #000000; font-weight: bold">in</span> <span style="color: #333333"><</span>module<span style="color: #333333">></span>
|
||||
<span style="color: #FF0000; font-weight: bold">OSError</span>: file error
|
||||
</pre></div>
|
||||
|
||||
|
||||
<p><a id='handling_exceptions'></a>
|
||||
<br>
|
||||
<br></p>
|
||||
|
||||
<h3>Handling exceptions</h3>
|
||||
|
||||
<p>[<a href="#py23_overview">back to Python 2.x vs 3.x overview</a>]</p>
|
||||
|
||||
<p>Also the handling of exceptions has slightly changed in Python 3. Now, we have to use the <code>as</code> keyword!</p>
|
||||
|
||||
<!-- HTML generated using hilite.me -->
|
||||
|
||||
|
||||
<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%"><span style="color: #888888"># Python 2</span>
|
||||
<span style="color: #333333">>>></span> <span style="color: #008800; font-weight: bold">try</span>:
|
||||
<span style="color: #333333">...</span> blabla
|
||||
<span style="color: #333333">...</span> <span style="color: #008800; font-weight: bold">except</span> <span style="color: #FF0000; font-weight: bold">NameError</span>, err:
|
||||
<span style="color: #333333">...</span> <span style="color: #007020">print</span> err, <span style="background-color: #fff0f0">'--> our error msg'</span>
|
||||
<span style="color: #333333">...</span>
|
||||
name <span style="background-color: #fff0f0">'blabla'</span> <span style="color: #000000; font-weight: bold">is</span> <span style="color: #000000; font-weight: bold">not</span> defined <span style="color: #333333">--></span> our error msg
|
||||
|
||||
<span style="color: #888888"># Python 3</span>
|
||||
<span style="color: #333333">>>></span> <span style="color: #008800; font-weight: bold">try</span>:
|
||||
<span style="color: #333333">...</span> blabla
|
||||
<span style="color: #333333">...</span> <span style="color: #008800; font-weight: bold">except</span> <span style="color: #FF0000; font-weight: bold">NameError</span> <span style="color: #008800; font-weight: bold">as</span> err:
|
||||
<span style="color: #333333">...</span> <span style="color: #007020">print</span>(err, <span style="background-color: #fff0f0">'--> our error msg'</span>)
|
||||
<span style="color: #333333">...</span>
|
||||
name <span style="background-color: #fff0f0">'blabla'</span> <span style="color: #000000; font-weight: bold">is</span> <span style="color: #000000; font-weight: bold">not</span> defined <span style="color: #333333">--></span> our error msg
|
||||
</pre></div>
|
||||
|
||||
|
||||
<p><a id='next_next'></a>
|
||||
<br>
|
||||
<br></p>
|
||||
|
||||
<h3>The <code>next()</code> function and <code>.next()</code> method</h3>
|
||||
|
||||
<p>[<a href="#py23_overview">back to Python 2.x vs 3.x overview</a>]</p>
|
||||
|
||||
<p>Where you can use both function and method in Python 2.7.5, the <code>next()</code> function is all that remain in Python 3!</p>
|
||||
|
||||
<!-- HTML generated using hilite.me -->
|
||||
|
||||
|
||||
<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%"><span style="color: #888888"># Python 2</span>
|
||||
<span style="color: #333333">>>></span> my_generator <span style="color: #333333">=</span> (letter <span style="color: #008800; font-weight: bold">for</span> letter <span style="color: #000000; font-weight: bold">in</span> <span style="background-color: #fff0f0">'abcdefg'</span>)
|
||||
<span style="color: #333333">>>></span> my_generator<span style="color: #333333">.</span>next()
|
||||
<span style="background-color: #fff0f0">'a'</span>
|
||||
<span style="color: #333333">>>></span> <span style="color: #007020">next</span>(my_generator)
|
||||
<span style="background-color: #fff0f0">'b'</span>
|
||||
|
||||
<span style="color: #888888"># Python 3</span>
|
||||
<span style="color: #333333">>>></span> my_generator <span style="color: #333333">=</span> (letter <span style="color: #008800; font-weight: bold">for</span> letter <span style="color: #000000; font-weight: bold">in</span> <span style="background-color: #fff0f0">'abcdefg'</span>)
|
||||
<span style="color: #333333">>>></span> <span style="color: #007020">next</span>(my_generator)
|
||||
<span style="background-color: #fff0f0">'a'</span>
|
||||
<span style="color: #333333">>>></span> my_generator<span style="color: #333333">.</span>next()
|
||||
Traceback (most recent call last):
|
||||
File <span style="background-color: #fff0f0">"<stdin>"</span>, line <span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #000000; font-weight: bold">in</span> <span style="color: #333333"><</span>module<span style="color: #333333">></span>
|
||||
<span style="color: #FF0000; font-weight: bold">AttributeError</span>: <span style="background-color: #fff0f0">'generator'</span> <span style="color: #007020">object</span> has no attribute <span style="background-color: #fff0f0">'next'</span>
|
||||
</pre></div>
|
||||
|
||||
|
||||
<p><a id='loop_leak'></a>
|
||||
<br>
|
||||
<br></p>
|
||||
|
||||
<h3>In Python 3.x for-loop variables don't leak into the global namespace anymore</h3>
|
||||
|
||||
<p>[<a href="#py23_overview">back to Python 2.x vs 3.x overview</a>]</p>
|
||||
|
||||
<p>This goes back to a change that was made in Python 3.x and is described in <a href="https://docs.python.org/3/whatsnew/3.0.html">What’s New In Python 3.0</a> as follows:</p>
|
||||
|
||||
<p>"List comprehensions no longer support the syntactic form <code>[... for var in item1, item2, ...]</code>. Use <code>[... for var in (item1, item2, ...)]</code> instead. Also note that list comprehensions have different semantics: they are closer to syntactic sugar for a generator expression inside a <code>list()</code> constructor, and in particular the loop control variables are no longer leaked into the surrounding scope."</p>
|
||||
|
||||
<p><br>
|
||||
<code>[In:]</code></p>
|
||||
|
||||
<!-- HTML generated using hilite.me -->
|
||||
|
||||
|
||||
<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%"><span style="color: #008800; font-weight: bold">from</span> <span style="color: #0e84b5; font-weight: bold">platform</span> <span style="color: #008800; font-weight: bold">import</span> python_version
|
||||
<span style="color: #007020">print</span>(<span style="background-color: #fff0f0">'This code cell was executed in Python'</span>, python_version())
|
||||
|
||||
i <span style="color: #333333">=</span> <span style="color: #0000DD; font-weight: bold">1</span>
|
||||
<span style="color: #007020">print</span>([i <span style="color: #008800; font-weight: bold">for</span> i <span style="color: #000000; font-weight: bold">in</span> <span style="color: #007020">range</span>(<span style="color: #0000DD; font-weight: bold">5</span>)])
|
||||
<span style="color: #007020">print</span>(i, <span style="background-color: #fff0f0">'-> i in global'</span>)
|
||||
</pre></div>
|
||||
|
||||
|
||||
<p><br>
|
||||
<code>[Out:]</code></p>
|
||||
|
||||
<!-- HTML generated using hilite.me -->
|
||||
|
||||
|
||||
<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%">This code cell was executed in Python 3.3.5
|
||||
[0, 1, 2, 3, 4]
|
||||
1 -> i in global
|
||||
</pre></div>
|
||||
|
||||
|
||||
<p><br>
|
||||
<br>
|
||||
<br>
|
||||
<code>[In:]</code></p>
|
||||
|
||||
<!-- HTML generated using hilite.me -->
|
||||
|
||||
|
||||
<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%"><span style="color: #008800; font-weight: bold">from</span> <span style="color: #0e84b5; font-weight: bold">platform</span> <span style="color: #008800; font-weight: bold">import</span> python_version
|
||||
<span style="color: #007020">print</span> <span style="background-color: #fff0f0">'This code cell was executed in Python'</span>, python_version()
|
||||
|
||||
i <span style="color: #333333">=</span> <span style="color: #0000DD; font-weight: bold">1</span>
|
||||
<span style="color: #007020">print</span> [i <span style="color: #008800; font-weight: bold">for</span> i <span style="color: #000000; font-weight: bold">in</span> <span style="color: #007020">range</span>(<span style="color: #0000DD; font-weight: bold">5</span>)]
|
||||
<span style="color: #007020">print</span> i, <span style="background-color: #fff0f0">'-> i in global'</span>
|
||||
</pre></div>
|
||||
|
||||
|
||||
<p><br>
|
||||
<code>[Out:]</code></p>
|
||||
|
||||
<!-- HTML generated using hilite.me -->
|
||||
|
||||
|
||||
<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%">This code cell was executed in Python 2.7.6
|
||||
[0, 1, 2, 3, 4]
|
||||
4 -> i in global
|
||||
</pre></div>
|
||||
|
||||
|
||||
<p><a id='compare_unorder'></a>
|
||||
<br>
|
||||
<br></p>
|
||||
|
||||
<h4>Python 3.x prevents us from comparing unorderable types</h4>
|
||||
|
||||
<p>[<a href="#py23_overview">back to Python 2.x vs 3.x overview</a>]</p>
|
||||
|
||||
<p><br>
|
||||
<code>[In:]</code></p>
|
||||
|
||||
<!-- HTML generated using hilite.me -->
|
||||
|
||||
|
||||
<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%"><span style="color: #008800; font-weight: bold">from</span> <span style="color: #0e84b5; font-weight: bold">platform</span> <span style="color: #008800; font-weight: bold">import</span> python_version
|
||||
<span style="color: #007020">print</span> <span style="background-color: #fff0f0">'This code cell was executed in Python'</span>, python_version()
|
||||
|
||||
<span style="color: #007020">print</span> [<span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #0000DD; font-weight: bold">2</span>] <span style="color: #333333">></span> <span style="background-color: #fff0f0">'foo'</span>
|
||||
<span style="color: #007020">print</span> (<span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #0000DD; font-weight: bold">2</span>) <span style="color: #333333">></span> <span style="background-color: #fff0f0">'foo'</span>
|
||||
<span style="color: #007020">print</span> [<span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #0000DD; font-weight: bold">2</span>] <span style="color: #333333">></span> (<span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #0000DD; font-weight: bold">2</span>)
|
||||
</pre></div>
|
||||
|
||||
|
||||
<p><br>
|
||||
<code>[Out:]</code></p>
|
||||
|
||||
<!-- HTML generated using hilite.me -->
|
||||
|
||||
|
||||
<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%">This code cell was executed in Python 2.7.6
|
||||
False
|
||||
True
|
||||
False
|
||||
</pre></div>
|
||||
|
||||
|
||||
<p><br>
|
||||
<br>
|
||||
<br></p>
|
||||
|
||||
<p><code>[In:]</code></p>
|
||||
|
||||
<!-- HTML generated using hilite.me -->
|
||||
|
||||
|
||||
<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%"><span style="color: #008800; font-weight: bold">from</span> <span style="color: #0e84b5; font-weight: bold">platform</span> <span style="color: #008800; font-weight: bold">import</span> python_version
|
||||
<span style="color: #007020">print</span>(<span style="background-color: #fff0f0">'This code cell was executed in Python'</span>, python_version())
|
||||
|
||||
<span style="color: #007020">print</span>([<span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #0000DD; font-weight: bold">2</span>] <span style="color: #333333">></span> <span style="background-color: #fff0f0">'foo'</span>)
|
||||
<span style="color: #007020">print</span>((<span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #0000DD; font-weight: bold">2</span>) <span style="color: #333333">></span> <span style="background-color: #fff0f0">'foo'</span>)
|
||||
<span style="color: #007020">print</span>([<span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #0000DD; font-weight: bold">2</span>] <span style="color: #333333">></span> (<span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #0000DD; font-weight: bold">2</span>))
|
||||
</pre></div>
|
||||
|
||||
|
||||
<p><code>[Out:]</code></p>
|
||||
|
||||
<!-- HTML generated using hilite.me -->
|
||||
|
||||
|
||||
<div style="background: #ffffff; overflow:auto;width:auto;border:solid gray;border-width:.1em .1em .1em .8em;padding:.2em .6em;"><pre style="margin: 0; line-height: 125%">This code cell was executed <span style="color: #000000; font-weight: bold">in</span> Python <span style="color: #6600EE; font-weight: bold">3.3</span><span style="color: #333333">.</span><span style="color: #0000DD; font-weight: bold">5</span>
|
||||
<span style="color: #333333">---------------------------------------------------------------------------</span>
|
||||
<span style="color: #FF0000; font-weight: bold">TypeError</span> Traceback (most recent call last)
|
||||
<span style="color: #333333"><</span>ipython<span style="color: #333333">-</span><span style="color: #007020">input</span><span style="color: #333333">-</span><span style="color: #0000DD; font-weight: bold">3</span><span style="color: #333333">-</span><span style="color: #0000DD; font-weight: bold">1</span>d774c677f73<span style="color: #333333">></span> <span style="color: #000000; font-weight: bold">in</span> <span style="color: #333333"><</span>module<span style="color: #333333">></span>()
|
||||
<span style="color: #0000DD; font-weight: bold">2</span> <span style="color: #007020">print</span>(<span style="background-color: #fff0f0">'This code cell was executed in Python'</span>, python_version())
|
||||
<span style="color: #0000DD; font-weight: bold">3</span>
|
||||
<span style="color: #333333">----></span> <span style="color: #0000DD; font-weight: bold">4</span> [<span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #0000DD; font-weight: bold">2</span>] <span style="color: #333333">></span> <span style="background-color: #fff0f0">'foo'</span>
|
||||
<span style="color: #0000DD; font-weight: bold">5</span> (<span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #0000DD; font-weight: bold">2</span>) <span style="color: #333333">></span> <span style="background-color: #fff0f0">'foo'</span>
|
||||
<span style="color: #0000DD; font-weight: bold">6</span> [<span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #0000DD; font-weight: bold">2</span>] <span style="color: #333333">></span> (<span style="color: #0000DD; font-weight: bold">1</span>, <span style="color: #0000DD; font-weight: bold">2</span>)
|
||||
|
||||
<span style="color: #FF0000; font-weight: bold">TypeError</span>: unorderable types: <span style="color: #007020">list</span>() <span style="color: #333333">></span> <span style="color: #007020">str</span>()
|
||||
</pre></div>
|
||||
|
||||
</body>
|
||||
</html>
|
||||
Loading…
Reference in New Issue
Block a user