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<div class="section" id="interactive-input-editing-and-history-substitution">
<span id="tut-interacting"></span><h1>13. Interactive Input Editing and History Substitution<a class="headerlink" href="#interactive-input-editing-and-history-substitution" title="Permalink to this headline">¶</a></h1>
<p>Some versions of the Python interpreter support editing of the current input
line and history substitution, similar to facilities found in the Korn shell and
the GNU Bash shell. This is implemented using the <a class="reference external" href="http://tiswww.case.edu/php/chet/readline/rltop.html">GNU Readline</a> library,
which supports Emacs-style and vi-style editing. This library has its own
documentation which I won’t duplicate here; however, the basics are easily
explained. The interactive editing and history described here are optionally
available in the Unix and Cygwin versions of the interpreter.</p>
<p>This chapter does <em>not</em> document the editing facilities of Mark Hammond’s
PythonWin package or the Tk-based environment, IDLE, distributed with Python.
The command line history recall which operates within DOS boxes on NT and some
other DOS and Windows flavors is yet another beast.</p>
<div class="section" id="line-editing">
<span id="tut-lineediting"></span><h2>13.1. Line Editing<a class="headerlink" href="#line-editing" title="Permalink to this headline">¶</a></h2>
<p>If supported, input line editing is active whenever the interpreter prints a
primary or secondary prompt. The current line can be edited using the
conventional Emacs control characters. The most important of these are:
<tt class="docutils literal"><span class="pre">C-A</span></tt> (Control-A) moves the cursor to the beginning of the line, <tt class="docutils literal"><span class="pre">C-E</span></tt>
to the end, <tt class="docutils literal"><span class="pre">C-B</span></tt> moves it one position to the left, <tt class="docutils literal"><span class="pre">C-F</span></tt> to the
right. Backspace erases the character to the left of the cursor, <tt class="docutils literal"><span class="pre">C-D</span></tt> the
character to its right. <tt class="docutils literal"><span class="pre">C-K</span></tt> kills (erases) the rest of the line to the
right of the cursor, <tt class="docutils literal"><span class="pre">C-Y</span></tt> yanks back the last killed string.
<tt class="docutils literal"><span class="pre">C-underscore</span></tt> undoes the last change you made; it can be repeated for
cumulative effect.</p>
</div>
<div class="section" id="history-substitution">
<span id="tut-history"></span><h2>13.2. History Substitution<a class="headerlink" href="#history-substitution" title="Permalink to this headline">¶</a></h2>
<p>History substitution works as follows. All non-empty input lines issued are
saved in a history buffer, and when a new prompt is given you are positioned on
a new line at the bottom of this buffer. <tt class="docutils literal"><span class="pre">C-P</span></tt> moves one line up (back) in
the history buffer, <tt class="docutils literal"><span class="pre">C-N</span></tt> moves one down. Any line in the history buffer
can be edited; an asterisk appears in front of the prompt to mark a line as
modified. Pressing the <tt class="docutils literal"><span class="pre">Return</span></tt> key passes the current line to the
interpreter. <tt class="docutils literal"><span class="pre">C-R</span></tt> starts an incremental reverse search; <tt class="docutils literal"><span class="pre">C-S</span></tt> starts
a forward search.</p>
</div>
<div class="section" id="key-bindings">
<span id="tut-keybindings"></span><h2>13.3. Key Bindings<a class="headerlink" href="#key-bindings" title="Permalink to this headline">¶</a></h2>
<p>The key bindings and some other parameters of the Readline library can be
customized by placing commands in an initialization file called
<tt class="docutils literal"><span class="pre">~/.inputrc</span></tt>. Key bindings have the form</p>
<div class="highlight-python"><pre>key-name: function-name</pre>
</div>
<p>or</p>
<div class="highlight-python"><pre>"string": function-name</pre>
</div>
<p>and options can be set with</p>
<div class="highlight-python"><pre>set option-name value</pre>
</div>
<p>For example:</p>
<div class="highlight-python"><pre># I prefer vi-style editing:
set editing-mode vi
# Edit using a single line:
set horizontal-scroll-mode On
# Rebind some keys:
Meta-h: backward-kill-word
"\C-u": universal-argument
"\C-x\C-r": re-read-init-file</pre>
</div>
<p>Note that the default binding for <tt class="docutils literal"><span class="pre">Tab</span></tt> in Python is to insert a <tt class="docutils literal"><span class="pre">Tab</span></tt>
character instead of Readline’s default filename completion function. If you
insist, you can override this by putting</p>
<div class="highlight-python"><pre>Tab: complete</pre>
</div>
<p>in your <tt class="docutils literal"><span class="pre">~/.inputrc</span></tt>. (Of course, this makes it harder to type indented
continuation lines if you’re accustomed to using <tt class="docutils literal"><span class="pre">Tab</span></tt> for that purpose.)</p>
<p id="index-1092">Automatic completion of variable and module names is optionally available. To
enable it in the interpreter’s interactive mode, add the following to your
startup file: <a class="footnote-reference" href="#id2" id="id1">[1]</a></p>
<div class="highlight-python"><div class="highlight"><pre><span class="kn">import</span> <span class="nn">rlcompleter</span><span class="o">,</span> <span class="nn">readline</span>
<span class="n">readline</span><span class="o">.</span><span class="n">parse_and_bind</span><span class="p">(</span><span class="s">'tab: complete'</span><span class="p">)</span>
</pre></div>
</div>
<p>This binds the <tt class="docutils literal"><span class="pre">Tab</span></tt> key to the completion function, so hitting the
<tt class="docutils literal"><span class="pre">Tab</span></tt> key twice suggests completions; it looks at Python statement names,
the current local variables, and the available module names. For dotted
expressions such as <tt class="docutils literal"><span class="pre">string.a</span></tt>, it will evaluate the expression up to the
final <tt class="docutils literal"><span class="pre">'.'</span></tt> and then suggest completions from the attributes of the resulting
object. Note that this may execute application-defined code if an object with a
<a title="object.__getattr__" class="reference external" href="../reference/datamodel.html#object.__getattr__"><tt class="xref docutils literal"><span class="pre">__getattr__()</span></tt></a> method is part of the expression.</p>
<p>A more capable startup file might look like this example. Note that this
deletes the names it creates once they are no longer needed; this is done since
the startup file is executed in the same namespace as the interactive commands,
and removing the names avoids creating side effects in the interactive
environment. You may find it convenient to keep some of the imported modules,
such as <a title="Miscellaneous operating system interfaces." class="reference external" href="../library/os.html#module-os"><tt class="xref docutils literal"><span class="pre">os</span></tt></a>, which turn out to be needed in most sessions with the
interpreter.</p>
<div class="highlight-python"><div class="highlight"><pre><span class="c"># Add auto-completion and a stored history file of commands to your Python</span>
<span class="c"># interactive interpreter. Requires Python 2.0+, readline. Autocomplete is</span>
<span class="c"># bound to the Esc key by default (you can change it - see readline docs).</span>
<span class="c">#</span>
<span class="c"># Store the file in ~/.pystartup, and set an environment variable to point</span>
<span class="c"># to it: "export PYTHONSTARTUP=/home/user/.pystartup" in bash.</span>
<span class="c">#</span>
<span class="c"># Note that PYTHONSTARTUP does *not* expand "~", so you have to put in the</span>
<span class="c"># full path to your home directory.</span>
<span class="kn">import</span> <span class="nn">atexit</span>
<span class="kn">import</span> <span class="nn">os</span>
<span class="kn">import</span> <span class="nn">readline</span>
<span class="kn">import</span> <span class="nn">rlcompleter</span>
<span class="n">historyPath</span> <span class="o">=</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">expanduser</span><span class="p">(</span><span class="s">"~/.pyhistory"</span><span class="p">)</span>
<span class="k">def</span> <span class="nf">save_history</span><span class="p">(</span><span class="n">historyPath</span><span class="o">=</span><span class="n">historyPath</span><span class="p">):</span>
<span class="kn">import</span> <span class="nn">readline</span>
<span class="n">readline</span><span class="o">.</span><span class="n">write_history_file</span><span class="p">(</span><span class="n">historyPath</span><span class="p">)</span>
<span class="k">if</span> <span class="n">os</span><span class="o">.</span><span class="n">path</span><span class="o">.</span><span class="n">exists</span><span class="p">(</span><span class="n">historyPath</span><span class="p">):</span>
<span class="n">readline</span><span class="o">.</span><span class="n">read_history_file</span><span class="p">(</span><span class="n">historyPath</span><span class="p">)</span>
<span class="n">atexit</span><span class="o">.</span><span class="n">register</span><span class="p">(</span><span class="n">save_history</span><span class="p">)</span>
<span class="k">del</span> <span class="n">os</span><span class="p">,</span> <span class="n">atexit</span><span class="p">,</span> <span class="n">readline</span><span class="p">,</span> <span class="n">rlcompleter</span><span class="p">,</span> <span class="n">save_history</span><span class="p">,</span> <span class="n">historyPath</span>
</pre></div>
</div>
</div>
<div class="section" id="alternatives-to-the-interactive-interpreter">
<span id="tut-commentary"></span><h2>13.4. Alternatives to the Interactive Interpreter<a class="headerlink" href="#alternatives-to-the-interactive-interpreter" title="Permalink to this headline">¶</a></h2>
<p>This facility is an enormous step forward compared to earlier versions of the
interpreter; however, some wishes are left: It would be nice if the proper
indentation were suggested on continuation lines (the parser knows if an indent
token is required next). The completion mechanism might use the interpreter’s
symbol table. A command to check (or even suggest) matching parentheses,
quotes, etc., would also be useful.</p>
<p>One alternative enhanced interactive interpreter that has been around for quite
some time is <a class="reference external" href="http://ipython.scipy.org/">IPython</a>, which features tab completion, object exploration and
advanced history management. It can also be thoroughly customized and embedded
into other applications. Another similar enhanced interactive environment is
<a class="reference external" href="http://www.bpython-interpreter.org/">bpython</a>.</p>
<p class="rubric">Footnotes</p>
<table class="docutils footnote" frame="void" id="id2" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>Python will execute the contents of a file identified by the
<span class="target" id="index-1093"></span><a class="reference external" href="../using/cmdline.html#envvar-PYTHONSTARTUP"><strong class="xref">PYTHONSTARTUP</strong></a> environment variable when you start an interactive
interpreter.</td></tr>
</tbody>
</table>
</div>
</div>
</div>
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<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference external" href="#">13. Interactive Input Editing and History Substitution</a><ul>
<li><a class="reference external" href="#line-editing">13.1. Line Editing</a></li>
<li><a class="reference external" href="#history-substitution">13.2. History Substitution</a></li>
<li><a class="reference external" href="#key-bindings">13.3. Key Bindings</a></li>
<li><a class="reference external" href="#alternatives-to-the-interactive-interpreter">13.4. Alternatives to the Interactive Interpreter</a></li>
</ul>
</li>
</ul>
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title="next chapter">14. Floating Point Arithmetic: Issues and Limitations</a></p>
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