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‎.ipynb_checkpoints/not_so_obvious_python_stuff-checkpoint.ipynb‎

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{
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"metadata": {
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"name": "",
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"signature": "sha256:437ca0df14a08cf4fd584f91122010286928f287ae14c5ac1ce8230abe9dbf9a"
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},
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"nbformat": 3,
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"nbformat_minor": 0,
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"worksheets": [
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{
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"cells": [
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Sebastian Raschka \n",
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"last updated: 04/15/2014"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"All code was executed in Python 3.4"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"# A collection of not so obvious Python stuff you should know!"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"# Sections\n",
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"- [The C3 class resolution algorithm for multiple class inheritance](#c3_class_res)\n",
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"- [The behavior of += for lists](#pm_in_lists)\n",
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"- [`True` and `False` in the datetime module](#datetime_module)\n",
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"- [Some Python Oddity that I cannot explain, yet](#python_oddity)\n",
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"- [Shallow vs. deep copies if list contains other structures and objects](#shallow_vs_deep)\n",
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"- [Picking True values from and and or expressions](#false_true_expressions)"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"<br>\n",
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"<br>\n",
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"<a name='c3_class_res'></a>\n",
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"## The C3 class resolution algorithm for multiple class inheritance"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"If we are dealing with multiple inheritance, according to the newer C3 class resolution algorithm, the following applies: \"class A should be checked before class B\".\n",
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"\n",
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"If you want to learn more, please read the [original blog](http://python-history.blogspot.ru/2010/06/method-resolution-order.html) post by Guido van Rossum.\n",
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"\n",
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"(Original source: [http://gistroll.com/rolls/21/horizontal_assessments/new](http://gistroll.com/rolls/21/horizontal_assessments/new))"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"class A(object):\n",
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" def foo(self):\n",
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" print(\"class A\")\n",
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"\n",
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"class B(object):\n",
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" def foo(self):\n",
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" print(\"class B\")\n",
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"\n",
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"class C(A, B):\n",
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" pass\n",
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"\n",
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"C().foo()"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"class A\n"
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]
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}
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],
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"prompt_number": 2
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"<br>\n",
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"<br>\n",
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"<a name='pm_in_lists'></a>\n",
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"## The behavior of `+=` for lists"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"If we are using the `+=` operator on lists, we extend the list by modifying the object directly. However, if we use the assigment via `my_list = my_list + ...`, we create a new list object, which can be demonstrated by the following code:\n",
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"\n",
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"(Original source: [http://stackoverflow.com/questions/2347265/why-does-behave-unexpectedly-on-lists](http://stackoverflow.com/questions/2347265/why-does-behave-unexpectedly-on-lists))"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"list_a = []\n",
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"print('ID of list_a', id(list_a))\n",
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"list_a += [1]\n",
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"print('ID of list_a after `+= [1]`', id(list_a))\n",
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"list_a = list_a + [2]\n",
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"print('ID of list_a after `list_a = list_a + [2]`', id(list_a))"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"ID of list_a 4356439144\n",
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"ID of list_a after `+= [1]` 4356439144\n",
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"ID of list_a after `list_a = list_a + [2]` 4356446112\n"
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]
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}
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],
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"prompt_number": 3
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"<br>\n",
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"<br>\n",
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"<a name='datetime_module'></a>\n",
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"## `True` and `False` in the datetime module\n",
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"\n",
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"\"it often comes as a big surprise for programmers to find (sometimes by way of a hard-to-reproduce bug) that,\n",
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"unlike any other time value, midnight (i.e. datetime.time(0,0,0)) is False.\n",
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"A long discussion on the python-ideas mailing list shows that, while surprising,\n",
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"that behavior is desirable\u2014at least in some quarters.\"\n",
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"\n",
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"(Original source: [http://lwn.net/SubscriberLink/590299/bf73fe823974acea/](http://lwn.net/SubscriberLink/590299/bf73fe823974acea/))"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"import datetime\n",
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"\n",
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"print('\"datetime.time(0,0,0)\" (Midnight) evaluates to', bool(datetime.time(0,0,0)))\n",
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"\n",
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"print('\"datetime.time(1,0,0)\" (1 am) evaluates to', bool(datetime.time(1,0,0)))"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"\"datetime.time(0,0,0)\" (Midnight) evaluates to False\n",
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"\"datetime.time(1,0,0)\" (1 am) evaluates to True\n"
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]
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}
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],
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"prompt_number": 4
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"<br>\n",
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"<br>\n",
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"<a name='python_oddity'></a>\n",
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"## Some Python Oddity that I cannot explain, yet"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"a = 1\n",
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"b = 1\n",
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"print('a is b', bool(a is b))\n",
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"True\n",
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"\n",
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"a = 999\n",
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"b = 999\n",
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"print('a is b', bool(a is b))"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"a is b True\n",
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"a is b False\n"
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]
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}
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],
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"prompt_number": 5
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"<br>\n",
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"<br>\n",
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"<a name='shallow_vs_deep'></a>\n",
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"## Shallow vs. deep copies if list contains other structures and objects\n",
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"\n",
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"List modification of the original list does affect shallow copies, but not deep copies if the list contains compound objects."
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"from copy import deepcopy\n",
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"\n",
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"my_first_list = [[1],[2]]\n",
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"my_second_list = [[1],[2]]\n",
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"print('my_first_list == my_second_list:', my_first_list == my_second_list)\n",
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"print('my_first_list is my_second_list:', my_first_list is my_second_list)\n",
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"\n",
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"my_third_list = my_first_list\n",
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"print('my_first_list == my_third_list:', my_first_list == my_third_list)\n",
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"print('my_first_list is my_third_list:', my_first_list is my_third_list)\n",
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"\n",
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"my_shallow_copy = my_first_list[:]\n",
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"print('my_first_list == my_shallow_copy:', my_first_list == my_shallow_copy)\n",
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"print('my_first_list is my_shallow_copy:', my_first_list is my_shallow_copy)\n",
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"\n",
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"my_deep_copy = deepcopy(my_first_list)\n",
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"print('my_first_list == my_deep_copy:', my_first_list == my_deep_copy)\n",
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"print('my_first_list is my_deep_copy:', my_first_list is my_deep_copy)\n",
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"\n",
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"print('\\nmy_third_list:', my_third_list)\n",
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"print('my_shallow_copy:', my_shallow_copy)\n",
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"print('my_deep_copy:', my_deep_copy)\n",
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"\n",
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"my_first_list[0][0] = 2\n",
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"print('after setting \"my_first_list[0][0] = 2\"')\n",
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"print('my_third_list:', my_third_list)\n",
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"print('my_shallow_copy:', my_shallow_copy)\n",
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"print('my_deep_copy:', my_deep_copy)"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"my_first_list == my_second_list: True\n",
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"my_first_list is my_second_list: False\n",
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"my_first_list == my_third_list: True\n",
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"my_first_list is my_third_list: True\n",
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"my_first_list == my_shallow_copy: True\n",
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"my_first_list is my_shallow_copy: False\n",
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"my_first_list == my_deep_copy: True\n",
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"my_first_list is my_deep_copy: False\n",
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"\n",
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"my_third_list: [[1], [2]]\n",
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"my_shallow_copy: [[1], [2]]\n",
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"my_deep_copy: [[1], [2]]\n",
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"after setting \"my_first_list[0][0] = 2\"\n",
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"my_third_list: [[2], [2]]\n",
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"my_shallow_copy: [[2], [2]]\n",
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"my_deep_copy: [[1], [2]]\n"
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]
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}
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],
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"prompt_number": 7
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"<br>\n",
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"<br>\n",
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"<a name='false_true_expressions'></a>\n",
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"## Picking `True` values from `and` and `or` expressions\n",
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"\n",
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"If both values of in a `or` expression are True, Python will select the first one, and the second one in `and` expressions\n",
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"\n",
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"(Original source: [http://gistroll.com/rolls/21/horizontal_assessments/new](http://gistroll.com/rolls/21/horizontal_assessments/new))"
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]
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},
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{
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"cell_type": "code",
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"collapsed": false,
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"input": [
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"result = (2 or 3) * (5 and 7)\n",
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"print('2 * 7 =', result)"
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],
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"language": "python",
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"metadata": {},
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"outputs": [
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{
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"output_type": "stream",
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"stream": "stdout",
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"text": [
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"2 * 7 = 14\n"
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]
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}
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],
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"prompt_number": 9
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}
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],
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"metadata": {}
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}
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]
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}

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