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<h1>Lesson 17: Lambda Functions</h1>
<div class="meta"><span class="badge">Intermediate</span><span>&#9200; 7 min</span></div>

<p>A <strong>lambda</strong> is a small, anonymous function defined with the <code>lambda</code> keyword. Unlike regular functions defined with <code>def</code>, lambdas are written in a single expression, have no name, and cannot contain statements or annotations. Syntax: <code>lambda arguments: expression</code>. The expression is evaluated and returned automatically — no <code>return</code> keyword needed: <code>lambda x, y: x + y</code> creates a function that adds two numbers.</p>

<pre><span class="cm"># Lambda = anonymous function</span>
square = <span class="kw">lambda</span> <span class="va">x</span>: x <span class="op">**</span> <span class="num">2</span>
<span class="kw">print</span>(square(<span class="num">5</span>))

add = <span class="kw">lambda</span> <span class="va">a</span>, <span class="va">b</span>: a <span class="op">+</span> b
<span class="kw">print</span>(add(<span class="num">3</span>, <span class="num">7</span>))

<span class="cm"># Use with sorted</span>
points = [(<span class="num">1</span>, <span class="num">5</span>), (<span class="num">3</span>, <span class="num">2</span>), (<span class="num">2</span>, <span class="num">8</span>)]
points.<span class="fn">sort</span>(key=<span class="kw">lambda</span> <span class="va">p</span>: p[<span class="num">1</span>])
<span class="kw">print</span>(<span class="str">"Sorted by y:"</span>, points)</pre>

<p>Lambdas shine when you need a simple, throwaway function, especially as an argument to higher-order functions:</p>
<ul>
<li><strong>sorted() with a custom key:</strong> <code>students.sort(key=lambda s: s["grade"])</code> — sort a list of dicts by the "grade" field. Also works with <code>max()</code> and <code>min()</code>: <code>max(points, key=lambda p: p[1])</code> finds the tuple with the largest y-coordinate.</li>
<li><strong>map():</strong> <code>list(map(lambda x: x * 1.15, prices))</code> — apply a 15% tax to each price.</li>
<li><strong>filter():</strong> <code>list(filter(lambda x: len(x) > 5, words))</code> — keep only words longer than 5 characters.</li>
</ul>

<h2>When NOT to use lambda:</h2>
<p>If the logic requires statements (if/elif/else chains, loops, try/except, variable assignments), you cannot use lambda — use <code>def</code>. If the function is used in multiple places, define it once with <code>def</code> to avoid repetition. If the lambda expression is too long to read comfortably on one line, use <code>def</code>. As a rule of thumb: if you need more than a single expression or the expression is longer than ~80 characters, write a regular function.</p>

<h2>Lambda vs def comparison:</h2>
<p>Both create function objects that can be called the same way. <code>def</code> functions have a name (helpful in tracebacks), support docstrings, annotations, and multiple statements. Lambdas are limited to a single expression and have no name or docstring — if they raise an error, the traceback shows <code>&lt;lambda&gt;</code> which is less helpful for debugging. Performance-wise, they are identical — choice is purely about readability and capability.</p>

<h2>Common mistakes:</h2>
<p>Trying to use statements inside a lambda — <code>lambda x: return x + 1</code> is a <code>SyntaxError</code>. Using a lambda when <code>functools.partial</code> or a regular function would be clearer. Assigning a lambda to a variable (<code>add = lambda x, y: x + y</code>) is poor style — if you're going to name it, use <code>def</code> instead. That's what <code>def</code> is for.</p>

<h2>Best practices:</h2>
<p>Use lambdas only for simple, single-use operations. Prefer list comprehensions over <code>map()</code> with lambdas for readability. Use <code>key=</code> arguments in <code>sorted()</code>, <code>max()</code>, <code>min()</code> — these are the most common and idiomatic use cases. If the lambda body is complex, refactor it into a named function with a descriptive name.</p>

<h2>Real-world use case:</h2>
<p>Sorting a list of product objects by price descending: <code>products.sort(key=lambda p: p["price"], reverse=True)</code>. Finding the employee with the highest salary: <code>best = max(employees, key=lambda e: e.salary)</code>. Grouping data with <code>itertools.groupby</code>: <code>for city, group in groupby(people, key=lambda p: p["city"]):</code>.</p>

<div class="tip"><strong>Try it:</strong> Use <code>max()</code> with a lambda to find the longest word in a list: <code>words = ["cat", "elephant", "dog"]; print(max(words, key=lambda w: len(w)))</code>. Then use <code>sorted()</code> with a lambda that sorts by the last character of each word. Finally, compare the readability of a lambda vs a <code>def</code> for a multi-step key function.</div>

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