Merge pull request realpython#224 from Julian/gotchas

Started writing about common gotchas with two quick examples

Author: Kenneth Reitz

docs/contents.rst.inc

@@ -31,6 +31,7 @@ This part of the guide focuses on best practices for writing Python code.
    writing/style
    writing/documentation
    writing/tests
+   writing/gotchas
    writing/license
 
 

docs/writing/gotchas.rst

@@ -0,0 +1,124 @@
+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 potentially 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 once you're aware of the underlying cause for the surprise.
+
+
+.. _default_args:
+
+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.
+
+
+Late Binding Closures
+---------------------
+
+Another common source of confusion is the way Python binds its variables in
+closures (or in the surrounding global scope).
+
+**What You Wrote**
+
+.. code-block:: python
+
+    def create_adders():
+        return [lambda x : i * x for i in range(5)]
+
+**What You Might Have Expected to Happen**
+
+A list containing five functions that each have their own closed-over ``i``
+variable that multiplies their argument.
+
+**What Does Happen**
+
+Five functions are created, but all of them just multiply ``x`` by 4.
+
+Python's closures are *late binding*. This means that names within closures are
+looked up at the time the inner function is *called*.
+
+Here, whenever *any* of the returned functions are called, the value of ``i``
+is looked up in the surrounding scope at call time, when by then the loop has
+completed and ``i`` is left with its final value of 4.
+
+What's particularly nasty about this gotcha is the seemingly prevalent
+misinformation that this has something to do with ``lambda``\s in Python.
+Functions created with a ``lambda`` expression are in no way special, and in
+fact the same exact behavior is exhibited by just using an ordinary ``def``:
+
+.. code-block:: python
+
+    def create_adders():
+        for i in range(5):
+            def adder(x):
+                return i * x
+            yield adder
+
+**What You Should Do Instead**
+
+Well. Here the general solution is arguably a bit of a hack. Due to Python's
+afformentioned behavior concerning evaluating default arguments to functions
+(see :ref:`default_args`), you can create a closure that binds immediately to
+its arguments by using a default arg like so:
+
+.. code-block:: python
+
+    def create_adders():
+        return [lambda x, i=i : i * x for i in range(5)]
+
+**When the Gotcha Isn't a Gotcha**
+
+When you want your closures to behave this way. Late binding is good in lots of
+situations. Looping to create unique functions is unfortunately a case where
+they can cause hiccups.