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<div class="section" id="anonymous-functions-lambda">
<span id="anonymous-functions"></span><h1>Anonymous Functions: Lambda<a class="headerlink" href="#anonymous-functions-lambda" title="Permalink to this headline"></a></h1>
<p>The <code class="docutils literal notranslate"><span class="pre">lambda</span></code> keyword is a way to create a function on the fly, without giving it a name. Hence the term “anonymous” – it has no name.</p>
<div class="section" id="using-lambda">
<h2>Using <code class="docutils literal notranslate"><span class="pre">lambda</span></code><a class="headerlink" href="#using-lambda" title="Permalink to this headline"></a></h2>
<div class="section" id="structure">
<h3>Structure<a class="headerlink" href="#structure" title="Permalink to this headline"></a></h3>
<p>“lambda” is a keyword – it can not be used as a variable name, etc. A lambda expression looks like:</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">lambda</span> <span class="o"><</span><span class="n">parameters</span><span class="o">></span> <span class="p">:</span> <span class="o"><</span><span class="n">expression</span><span class="o">></span>
</pre></div>
</div>
<p>It is an expression – which means it evaluates to a value – which means that it can be assigned to a variable, put in a container (such as a list or dict) etc.</p>
<p>The parameters are just like function parameters – but without the parentheses. And they exist in the local scope of the lambda, and can thus be used in the expression.</p>
<p>The expression can be any expression using global variables or the parameters – but not a statement – anyone remember what the difference is?</p>
<p>An expression is something that evaluates to a value, and can be assigned to a variable etc. but things like <code class="docutils literal notranslate"><span class="pre">for</span></code> and <code class="docutils literal notranslate"><span class="pre">while</span></code> loops and <code class="docutils literal notranslate"><span class="pre">if...else</span></code> structures are NOT expressions, and thus can’t be put in a lambda.</p>
<p>In short, lambda is only for very simple functions with no real flow control.</p>
</div>
<div class="section" id="example">
<h3>Example<a class="headerlink" href="#example" title="Permalink to this headline"></a></h3>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [1]: </span><span class="n">f</span> <span class="o">=</span> <span class="k">lambda</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">:</span> <span class="n">x</span> <span class="o">+</span> <span class="n">y</span>
<span class="gp">In [2]: </span><span class="n">f</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">)</span>
<span class="gh">Out[2]: </span><span class="go">7</span>
</pre></div>
</div>
<p>This creates a function that takes two positional parameters, <code class="docutils literal notranslate"><span class="pre">x</span></code> and <code class="docutils literal notranslate"><span class="pre">y</span></code>, and returns their sum.</p>
<p>This is ALMOST the same as:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [3]: </span><span class="k">def</span> <span class="nf">f2</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">):</span>
<span class="gp"> ...: </span> <span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="n">y</span>
<span class="gp">In [4]: </span><span class="n">f2</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">)</span>
<span class="gh">Out[4]: </span><span class="go">7</span>
</pre></div>
</div>
<p>So what is the difference? Well, <code class="docutils literal notranslate"><span class="pre">lambda</span></code> creates an “anonymous” function – it doesn’t have a name:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [10]: </span><span class="n">f</span><span class="o">.</span><span class="vm">__name__</span>
<span class="gh">Out[10]: </span><span class="go">'<lambda>'</span>
</pre></div>
</div>
<p>All lambda functions are called <lambda>. But:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [11]: </span><span class="n">f2</span><span class="o">.</span><span class="vm">__name__</span>
<span class="gh">Out[11]: </span><span class="go">'f2'</span>
</pre></div>
</div>
<p>Regular “def” defined functions know their name.</p>
<p>So <code class="docutils literal notranslate"><span class="pre">lambda</span></code> creates a function, but it doesn’t have a name, and it can only have a single expression in it – no flow control.</p>
</div>
<div class="section" id="what-s-the-point">
<h3>What’s the point?<a class="headerlink" href="#what-s-the-point" title="Permalink to this headline"></a></h3>
<p>There is nothing you can do with lambda that you can’t do with regular <code class="docutils literal notranslate"><span class="pre">def</span></code> functions. But there are times when you just need a simple quick function that you don’t need to keep around or refer to again – so it’s a handy syntax for these situations.</p>
<p>A sorting key function is a great example:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [55]: </span><span class="n">lst</span> <span class="o">=</span> <span class="p">[(</span><span class="s2">"Chris"</span><span class="p">,</span><span class="s2">"Barker"</span><span class="p">),</span> <span class="p">(</span><span class="s2">"Fred"</span><span class="p">,</span> <span class="s2">"Jones"</span><span class="p">),</span> <span class="p">(</span><span class="s2">"Zola"</span><span class="p">,</span> <span class="s2">"Adams"</span><span class="p">)]</span>
<span class="gp">In [56]: </span><span class="n">lst</span><span class="o">.</span><span class="n">sort</span><span class="p">()</span>
<span class="gp">In [57]: </span><span class="n">lst</span>
<span class="gh">Out[57]: </span><span class="go">[('Chris', 'Barker'), ('Fred', 'Jones'), ('Zola', 'Adams')]</span>
</pre></div>
</div>
<p>Tuples are sorted, be default, by their first element. But what if you wanted to sort by the second (index 1) element – last name in this case:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [13]: </span><span class="k">def</span> <span class="nf">sort_key</span><span class="p">(</span><span class="n">item</span><span class="p">):</span>
<span class="go"> ...: return item[1]</span>
<span class="go"> ...:</span>
<span class="gp">In [14]: </span><span class="n">lst</span><span class="o">.</span><span class="n">sort</span><span class="p">(</span><span class="n">key</span><span class="o">=</span><span class="n">sort_key</span><span class="p">)</span>
<span class="gp">In [15]: </span><span class="n">lst</span>
<span class="gh">Out[15]: </span><span class="go">[('Zola', 'Adams'), ('Chris', 'Barker'), ('Fred', 'Jones')]</span>
</pre></div>
</div>
<p>straightforward enough. But kind a lot of extra code, eh? and now there is this function: “sort_key” hanging around. You could delete it: <code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">sort_key</span></code>, but that would be even more code.</p>
<p>But with a lambda, you simply define it inline:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [16]: </span><span class="n">lst</span> <span class="o">=</span> <span class="p">[(</span><span class="s2">"Chris"</span><span class="p">,</span><span class="s2">"Barker"</span><span class="p">),</span> <span class="p">(</span><span class="s2">"Fred"</span><span class="p">,</span> <span class="s2">"Jones"</span><span class="p">),</span> <span class="p">(</span><span class="s2">"Zola"</span><span class="p">,</span> <span class="s2">"Adams"</span><span class="p">)]</span>
<span class="gp">In [17]: </span><span class="n">lst</span><span class="o">.</span><span class="n">sort</span><span class="p">(</span><span class="n">key</span><span class="o">=</span><span class="k">lambda</span> <span class="n">x</span><span class="p">:</span> <span class="n">x</span><span class="p">[</span><span class="mi">1</span><span class="p">])</span>
<span class="gp">In [18]: </span><span class="n">lst</span>
<span class="gh">Out[18]: </span><span class="go">[('Zola', 'Adams'), ('Chris', 'Barker'), ('Fred', 'Jones')]</span>
</pre></div>
</div>
<p>Nice and compact and clear, with no extra names hanging around.</p>
<p>You’ll find them useful with things like <code class="docutils literal notranslate"><span class="pre">map</span></code>, <code class="docutils literal notranslate"><span class="pre">filter</span></code>, and <code class="docutils literal notranslate"><span class="pre">reduce</span></code>. as well.</p>
</div>
</div>
<div class="section" id="functions-as-first-class-objects">
<h2>Functions as first class objects<a class="headerlink" href="#functions-as-first-class-objects" title="Permalink to this headline"></a></h2>
<p>lambda functions are python objects; they can be stored in a list or other container:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [7]: </span><span class="n">l</span> <span class="o">=</span> <span class="p">[</span><span class="k">lambda</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">:</span> <span class="n">x</span><span class="o">+</span><span class="n">y</span><span class="p">]</span>
<span class="gp">In [8]: </span><span class="nb">type</span><span class="p">(</span><span class="n">l</span><span class="p">[</span><span class="mi">0</span><span class="p">])</span>
<span class="gh">Out[8]: </span><span class="go">function</span>
</pre></div>
</div>
<p>And you can call it by indexing the container:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [9]: </span><span class="n">l</span><span class="p">[</span><span class="mi">0</span><span class="p">](</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">)</span>
<span class="gh">Out[9]: </span><span class="go">7</span>
</pre></div>
</div>
<p>You can do that with “regular” functions too:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [12]: </span><span class="k">def</span> <span class="nf">fun</span><span class="p">(</span><span class="n">x</span><span class="p">,</span><span class="n">y</span><span class="p">):</span>
<span class="gp"> ....: </span> <span class="k">return</span> <span class="n">x</span><span class="o">+</span><span class="n">y</span>
<span class="gp"> ....:</span>
<span class="gp">In [13]: </span><span class="n">l</span> <span class="o">=</span> <span class="p">[</span><span class="n">fun</span><span class="p">]</span>
<span class="gp">In [14]: </span><span class="nb">type</span><span class="p">(</span><span class="n">l</span><span class="p">[</span><span class="mi">0</span><span class="p">])</span>
<span class="gh">Out[14]: </span><span class="go">function</span>
<span class="gp">In [15]: </span><span class="n">l</span><span class="p">[</span><span class="mi">0</span><span class="p">](</span><span class="mi">3</span><span class="p">,</span><span class="mi">4</span><span class="p">)</span>
<span class="gh">Out[15]: </span><span class="go">7</span>
</pre></div>
</div>
<p>If the goal is to have that little function in the list, and you don’t need to give it a name and/or reference it anywhere else, then lambda is a cleaner way to do it.</p>
<div class="section" id="lambda-and-keyword-arguments">
<h3>lambda and keyword arguments<a class="headerlink" href="#lambda-and-keyword-arguments" title="Permalink to this headline"></a></h3>
<p>lambda functions can take keyword arguments as well:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [20]: </span><span class="p">(</span><span class="k">lambda</span> <span class="n">x</span><span class="o">=</span><span class="kc">None</span><span class="p">:</span> <span class="n">x</span> <span class="o">*</span> <span class="mi">2</span><span class="p">)(</span><span class="n">x</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span>
<span class="gh">Out[20]: </span><span class="go">8</span>
</pre></div>
</div>
<p>Remember that default arguments get evaluated when the function is defined. This is the case with lambda as well. This can get you in trouble if you use a mutable in a function definition. But it also can be a handy way to “bake in” a value into a function to be used later:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [186]: </span><span class="n">l</span> <span class="o">=</span> <span class="p">[]</span>
<span class="gp">In [187]: </span><span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span>
<span class="go"> l.append(lambda x, e=i: x**e)</span>
</pre></div>
</div>
<p>This creates a list, in this case with three items. Each of those items is a function. Note that the lambda is called each time through the loop, so each one has that default parameter defined separately – and each time the default is set to the current value of <code class="docutils literal notranslate"><span class="pre">i</span></code> in the loop. So we get three functions, all the same except for the default value of <code class="docutils literal notranslate"><span class="pre">e</span></code> – that is, each function will raise the input value to a different power.</p>
<p>We can loop through that list, and call each function in turn with the same input value:</p>
<div class="highlight-ipython notranslate"><div class="highlight"><pre><span></span><span class="gp">In [25]: </span><span class="k">for</span> <span class="n">f</span> <span class="ow">in</span> <span class="n">func_list</span><span class="p">:</span>
<span class="go"> ...: print(f(2))</span>
<span class="go"> ...:</span>
<span class="go">1</span>
<span class="go">2</span>
<span class="go">4</span>
<span class="go">8</span>
</pre></div>
</div>
<p>and presto! 2 raised to the zeroth, then first, the second, … power.</p>
<p>This may seem pretty obscure, but it’s a handy way to auto-generate custom functions on the fly – like for GUI callbacks, for instance:</p>
<p><a class="reference external" href="https://wiki.wxpython.org/Passing%20Arguments%20to%20Callbacks">https://wiki.wxpython.org/Passing%20Arguments%20to%20Callbacks</a></p>
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