First gotcha.

docs/writing/gotchas.rst

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+Common Gotchas
+==============
+
+For the most part, Python aims to be a clean and consistent language that
+avoid surprises, but there are a few cases where newcomers to the language
+often get tripped up.
+
+Some of these are intentional but surprising. Some could arguably be considered
+language warts. In general though, what follows is a collection of potentially
+tricky behavior that might seem strange at first glance, but is generally
+sensible despite surprise after learning of its existence.
+
+
+Mutable Default Arguments
+-------------------------
+
+Seemingly the *most* common surprise new Python programmers encounter is
+Python's treatment of mutable default arguments in function definitions.
+
+**What You Wrote**
+
+.. code-block:: python
+
+    def append_to(element, to=[]):
+        to.append(element)
+        return to
+
+**What You Might Have Expected to Happen**
+
+A new list is created each time the function is called if a second argument
+isn't provided.
+
+**What Does Happen**
+
+A new list is created *once* when the function is defined, and the same list is
+used in each successive call.
+
+Python's default arguments are evaluated *once* when the function is defined,
+not each time the function is called (like it is in say, Ruby). This means that
+if you use a mutable default argument and mutate it, you *will* and have
+mutated that object for all future calls to the function as well.
+
+**What You Should Do Instead**
+
+Create a new object each time the function is called, by using a default arg to
+signal that no argument was provided (``None`` is often a good choice).
+
+.. code-block:: python
+
+    def append_to(element, to=None):
+        if to is None:
+            to = []
+        to.append(element)
+        return to
+
+
+**When the Gotcha Isn't a Gotcha**
+
+Sometimes you specifically can "exploit" (read: use as intended) this behavior
+to maintain state between calls of a function. This is often done when writing
+a caching function.