diff options
Diffstat (limited to 'Doc/faq/programming.rst')
| -rw-r--r-- | Doc/faq/programming.rst | 95 |
1 files changed, 78 insertions, 17 deletions
diff --git a/Doc/faq/programming.rst b/Doc/faq/programming.rst index d85563f..c30c2b6 100644 --- a/Doc/faq/programming.rst +++ b/Doc/faq/programming.rst @@ -292,9 +292,8 @@ What are the "best practices" for using import in a module? ----------------------------------------------------------- In general, don't use ``from modulename import *``. Doing so clutters the -importer's namespace. Some people avoid this idiom even with the few modules -that were designed to be imported in this manner. Modules designed in this -manner include :mod:`tkinter`, and :mod:`threading`. +importer's namespace, and makes it much harder for linters to detect undefined +names. Import modules at the top of a file. Doing so makes it clear what other modules your code requires and avoids questions of whether the module name is in scope. @@ -308,11 +307,6 @@ It's good practice if you import modules in the following order: directory) -- e.g. mx.DateTime, ZODB, PIL.Image, etc. 3. locally-developed modules -Never use relative package imports. If you're writing code that's in the -``package.sub.m1`` module and want to import ``package.sub.m2``, do not just -write ``from . import m2``, even though it's legal. Write ``from package.sub -import m2`` instead. See :pep:`328` for details. - It is sometimes necessary to move imports to a function or class to avoid problems with circular imports. Gordon McMillan says: @@ -343,14 +337,6 @@ module, but loading a module multiple times is virtually free, costing only a couple of dictionary lookups. Even if the module name has gone out of scope, the module is probably available in :data:`sys.modules`. -If only instances of a specific class use a module, then it is reasonable to -import the module in the class's ``__init__`` method and then assign the module -to an instance variable so that the module is always available (via that -instance variable) during the life of the object. Note that to delay an import -until the class is instantiated, the import must be inside a method. Putting -the import inside the class but outside of any method still causes the import to -occur when the module is initialized. - Why are default values shared between objects? ---------------------------------------------- @@ -401,7 +387,7 @@ requested again. This is called "memoizing", and can be implemented like this:: # Calculate the value result = ... expensive computation ... - _cache[(arg1, arg2)] = result # Store result in the cache + _cache[(arg1, arg2)] = result # Store result in the cache return result You could use a global variable containing a dictionary instead of the default @@ -448,6 +434,81 @@ arguments a function can accept. For example, given the function definition:: the values ``42``, ``314``, and ``somevar`` are arguments. +Why did changing list 'y' also change list 'x'? +------------------------------------------------ + +If you wrote code like:: + + >>> x = [] + >>> y = x + >>> y.append(10) + >>> y + [10] + >>> x + [10] + +you might be wondering why appending an element to ``y`` changed ``x`` too. + +There are two factors that produce this result: + +1) Variables are simply names that refer to objects. Doing ``y = x`` doesn't + create a copy of the list -- it creates a new variable ``y`` that refers to + the same object ``x`` refers to. This means that there is only one object + (the list), and both ``x`` and ``y`` refer to it. +2) Lists are :term:`mutable`, which means that you can change their content. + +After the call to :meth:`~list.append`, the content of the mutable object has +changed from ``[]`` to ``[10]``. Since both the variables refer to the same +object, using either name accesses the modified value ``[10]``. + +If we instead assign an immutable object to ``x``:: + + >>> x = 5 # ints are immutable + >>> y = x + >>> x = x + 1 # 5 can't be mutated, we are creating a new object here + >>> x + 6 + >>> y + 5 + +we can see that in this case ``x`` and ``y`` are not equal anymore. This is +because integers are :term:`immutable`, and when we do ``x = x + 1`` we are not +mutating the int ``5`` by incrementing its value; instead, we are creating a +new object (the int ``6``) and assigning it to ``x`` (that is, changing which +object ``x`` refers to). After this assignment we have two objects (the ints +``6`` and ``5``) and two variables that refer to them (``x`` now refers to +``6`` but ``y`` still refers to ``5``). + +Some operations (for example ``y.append(10)`` and ``y.sort()``) mutate the +object, whereas superficially similar operations (for example ``y = y + [10]`` +and ``sorted(y)``) create a new object. In general in Python (and in all cases +in the standard library) a method that mutates an object will return ``None`` +to help avoid getting the two types of operations confused. So if you +mistakenly write ``y.sort()`` thinking it will give you a sorted copy of ``y``, +you'll instead end up with ``None``, which will likely cause your program to +generate an easily diagnosed error. + +However, there is one class of operations where the same operation sometimes +has different behaviors with different types: the augmented assignment +operators. For example, ``+=`` mutates lists but not tuples or ints (``a_list ++= [1, 2, 3]`` is equivalent to ``a_list.extend([1, 2, 3])`` and mutates +``a_list``, whereas ``some_tuple += (1, 2, 3)`` and ``some_int += 1`` create +new objects). + +In other words: + +* If we have a mutable object (:class:`list`, :class:`dict`, :class:`set`, + etc.), we can use some specific operations to mutate it and all the variables + that refer to it will see the change. +* If we have an immutable object (:class:`str`, :class:`int`, :class:`tuple`, + etc.), all the variables that refer to it will always see the same value, + but operations that transform that value into a new value always return a new + object. + +If you want to know if two variables refer to the same object or not, you can +use the :keyword:`is` operator, or the built-in function :func:`id`. + + How do I write a function with output parameters (call by reference)? --------------------------------------------------------------------- |