mirror of
https://github.com/rasbt/python_reference.git
synced 2024-10-06 13:09:27 +00:00
types as variables
This commit is contained in:
rasbt
parent
c561c92eea
commit
95227cb057
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@ -1,7 +1,7 @@
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{
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"metadata": {
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"name": "",
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"signature": "sha256:d8ba69c66769cf62e5201b70ed7d717913017f6f09492848ce164b50068bd2ba"
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"signature": "sha256:5e808c95d2003c791e04a26d378bc816b2010a59171244a18d73487060d9e029"
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},
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"nbformat": 3,
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"nbformat_minor": 0,
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@ -79,7 +79,8 @@
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"- [Creating copies of mutable objects](#copy_mutable)\n",
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"- [Key differences between Python 2 and 3](#python_differences)\n",
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"- [Function annotations - What are those `->`'s in my Python code?](#function_annotation)\n",
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"- [Abortive statements in `finally` blocks](#finally_blocks)"
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"- [Abortive statements in `finally` blocks](#finally_blocks)\n",
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"- [Assigning types to variables as values](#variable_types)"
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]
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},
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{
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@ -922,7 +923,7 @@
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Chicken or egg? In the history of Python (Python 2.2 to be specific) truth values were implemented via 1 and 0 (similar to the old C), to avoid syntax error in old (but perfectly working) code, `bool` was added as a subclass of `int` in Python 2.3.\n",
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"Chicken or egg? In the history of Python (Python 2.2 to be specific) truth values were implemented via 1 and 0 (similar to the old C). In order to avoid syntax errors in old (but perfectly working) Python code, `bool` was added as a subclass of `int` in Python 2.3.\n",
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"\n",
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"Original source: [http://www.peterbe.com/plog/bool-is-int](http://www.peterbe.com/plog/bool-is-int)"
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]
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@ -933,7 +934,7 @@
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"input": [
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"print('isinstance(True, int):', isinstance(True, int))\n",
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"print('True + True:', True + True)\n",
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"print('3*True:', 3*True)\n",
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"print('3*True + True:', 3*True + True)\n",
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"print('3*True - False:', 3*True - False)\n"
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],
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"language": "python",
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@ -945,12 +946,12 @@
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"text": [
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"isinstance(True, int): True\n",
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"True + True: 2\n",
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"3*True: 3\n",
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"3*True + True: 4\n",
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"3*True - False: 3\n"
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]
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}
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],
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"prompt_number": 16
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"prompt_number": 28
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},
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{
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"cell_type": "markdown",
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@ -979,11 +980,11 @@
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Remember the [\"consuming generators\"](consuming_generators)? This example is somewhat related, but the result might still come unexpected. \n",
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"Remember the section about the [\"consuming generators\"](consuming_generators)? This example is somewhat related, but the result might still come unexpected. \n",
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"\n",
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"(Original source: [http://openhome.cc/eGossip/Blog/UnderstandingLambdaClosure3.html](http://openhome.cc/eGossip/Blog/UnderstandingLambdaClosure3.html))\n",
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"\n",
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"In the first example below, where we call a `lambda` function in a list comprehension, the value `i` is dereferenced every time we call `lambda` within the scope of the list comprehension. Since the list is already constructed when we `for-loop` through the list, it is set to the last value 4."
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"In the first example below, we call a `lambda` function in a list comprehension, and the value `i` will be dereferenced every time we call `lambda` within the scope of the list comprehension. Since the list is already constructed when we `for-loop` through the list, it will be set to the last value 4."
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]
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},
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{
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@ -1009,7 +1010,7 @@
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]
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}
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],
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"prompt_number": 11
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"prompt_number": 29
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},
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{
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"cell_type": "markdown",
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@ -1041,7 +1042,7 @@
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]
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}
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],
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"prompt_number": 9
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"prompt_number": 30
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},
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{
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"cell_type": "markdown",
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@ -1106,7 +1107,7 @@
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]
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}
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],
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"prompt_number": 33
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"prompt_number": 31
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},
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{
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"cell_type": "markdown",
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@ -1484,8 +1485,8 @@
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Not, really (or significantly, see the benchmarks below). So what's the reason to prefer one over the other?\n",
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"- use lists if you want to use list methods \n",
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"\"List comprehensions are fast, but generators are faster!?\" - No, not really (or significantly, see the benchmarks below). So what's the reason to prefer one over the other?\n",
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"- use lists if you want to use the plethora of list methods \n",
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"- use generators when you are dealing with huge collections to avoid memory issues"
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]
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},
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@ -1699,7 +1700,7 @@
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"It can be really dangerous to modify a list when iterating through - it is a very common pitfall that can cause unintended behavior! \n",
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"It can be really dangerous to modify a list when iterating through it - this is a very common pitfall that can cause unintended behavior! \n",
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"Look at the following examples, and for a fun exercise: try to figure out what is going on before you skip to the solution!"
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]
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},
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@ -2865,6 +2866,78 @@
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"Here, the abortive `return` statement in the `finally` block simply overrules the `return` in the `try` block, since **`finally` is guaranteed to always be executed.** So, be careful using abortive statements in `finally` blocks!"
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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=\"variable_types\">"
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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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"#Assigning types to variables as values"
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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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"I am not yet sure in which context this can be useful, but it is a nice fun fact to know that we can assign types as values to variables."
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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_var = str\n",
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"a_var(123)"
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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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"metadata": {},
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"output_type": "pyout",
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"prompt_number": 1,
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"text": [
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"'123'"
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]
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}
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],
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"prompt_number": 1
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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 random import choice\n",
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"\n",
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"a, b, c = float, int, str\n",
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"for i in range(5):\n",
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" j = choice([a,b,c])(i)\n",
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" print(j, type(j))"
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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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"0 <class 'int'>\n",
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"1 <class 'int'>\n",
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"2.0 <class 'float'>\n",
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"3 <class 'str'>\n",
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"4 <class 'int'>\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": "code",
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"collapsed": false,
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@ -1,7 +1,7 @@
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{
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"metadata": {
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"name": "",
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"signature": "sha256:d8ba69c66769cf62e5201b70ed7d717913017f6f09492848ce164b50068bd2ba"
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"signature": "sha256:5e808c95d2003c791e04a26d378bc816b2010a59171244a18d73487060d9e029"
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},
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"nbformat": 3,
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"nbformat_minor": 0,
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@ -79,7 +79,8 @@
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"- [Creating copies of mutable objects](#copy_mutable)\n",
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"- [Key differences between Python 2 and 3](#python_differences)\n",
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"- [Function annotations - What are those `->`'s in my Python code?](#function_annotation)\n",
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"- [Abortive statements in `finally` blocks](#finally_blocks)"
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"- [Abortive statements in `finally` blocks](#finally_blocks)\n",
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"- [Assigning types to variables as values](#variable_types)"
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]
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},
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{
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@ -922,7 +923,7 @@
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Chicken or egg? In the history of Python (Python 2.2 to be specific) truth values were implemented via 1 and 0 (similar to the old C), to avoid syntax error in old (but perfectly working) code, `bool` was added as a subclass of `int` in Python 2.3.\n",
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"Chicken or egg? In the history of Python (Python 2.2 to be specific) truth values were implemented via 1 and 0 (similar to the old C). In order to avoid syntax errors in old (but perfectly working) Python code, `bool` was added as a subclass of `int` in Python 2.3.\n",
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"\n",
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"Original source: [http://www.peterbe.com/plog/bool-is-int](http://www.peterbe.com/plog/bool-is-int)"
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]
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@ -933,7 +934,7 @@
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"input": [
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"print('isinstance(True, int):', isinstance(True, int))\n",
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"print('True + True:', True + True)\n",
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"print('3*True:', 3*True)\n",
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"print('3*True + True:', 3*True + True)\n",
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"print('3*True - False:', 3*True - False)\n"
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],
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"language": "python",
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@ -945,12 +946,12 @@
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"text": [
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"isinstance(True, int): True\n",
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"True + True: 2\n",
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"3*True: 3\n",
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"3*True + True: 4\n",
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"3*True - False: 3\n"
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]
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}
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],
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"prompt_number": 16
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"prompt_number": 28
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},
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{
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"cell_type": "markdown",
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@ -979,11 +980,11 @@
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"cell_type": "markdown",
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"metadata": {},
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"source": [
|
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"Remember the [\"consuming generators\"](consuming_generators)? This example is somewhat related, but the result might still come unexpected. \n",
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"Remember the section about the [\"consuming generators\"](consuming_generators)? This example is somewhat related, but the result might still come unexpected. \n",
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"\n",
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"(Original source: [http://openhome.cc/eGossip/Blog/UnderstandingLambdaClosure3.html](http://openhome.cc/eGossip/Blog/UnderstandingLambdaClosure3.html))\n",
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"\n",
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"In the first example below, where we call a `lambda` function in a list comprehension, the value `i` is dereferenced every time we call `lambda` within the scope of the list comprehension. Since the list is already constructed when we `for-loop` through the list, it is set to the last value 4."
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"In the first example below, we call a `lambda` function in a list comprehension, and the value `i` will be dereferenced every time we call `lambda` within the scope of the list comprehension. Since the list is already constructed when we `for-loop` through the list, it will be set to the last value 4."
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]
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},
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{
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@ -1009,7 +1010,7 @@
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]
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}
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],
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"prompt_number": 11
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"prompt_number": 29
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},
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{
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"cell_type": "markdown",
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@ -1041,7 +1042,7 @@
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]
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}
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],
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"prompt_number": 9
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"prompt_number": 30
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},
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{
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"cell_type": "markdown",
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@ -1106,7 +1107,7 @@
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]
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}
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],
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"prompt_number": 33
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"prompt_number": 31
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},
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{
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"cell_type": "markdown",
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@ -1484,8 +1485,8 @@
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"Not, really (or significantly, see the benchmarks below). So what's the reason to prefer one over the other?\n",
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"- use lists if you want to use list methods \n",
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"\"List comprehensions are fast, but generators are faster!?\" - No, not really (or significantly, see the benchmarks below). So what's the reason to prefer one over the other?\n",
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"- use lists if you want to use the plethora of list methods \n",
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"- use generators when you are dealing with huge collections to avoid memory issues"
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]
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},
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@ -1699,7 +1700,7 @@
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"It can be really dangerous to modify a list when iterating through - it is a very common pitfall that can cause unintended behavior! \n",
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"It can be really dangerous to modify a list when iterating through it - this is a very common pitfall that can cause unintended behavior! \n",
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"Look at the following examples, and for a fun exercise: try to figure out what is going on before you skip to the solution!"
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]
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},
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@ -2865,6 +2866,78 @@
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"Here, the abortive `return` statement in the `finally` block simply overrules the `return` in the `try` block, since **`finally` is guaranteed to always be executed.** So, be careful using abortive statements in `finally` blocks!"
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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=\"variable_types\">"
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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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"#Assigning types to variables as values"
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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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"I am not yet sure in which context this can be useful, but it is a nice fun fact to know that we can assign types as values to variables."
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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_var = str\n",
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"a_var(123)"
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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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"metadata": {},
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"output_type": "pyout",
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"prompt_number": 1,
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"text": [
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"'123'"
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]
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}
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],
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"prompt_number": 1
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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 random import choice\n",
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"\n",
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"a, b, c = float, int, str\n",
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"for i in range(5):\n",
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" j = choice([a,b,c])(i)\n",
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" print(j, type(j))"
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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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"0 <class 'int'>\n",
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"1 <class 'int'>\n",
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"2.0 <class 'float'>\n",
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"3 <class 'str'>\n",
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"4 <class 'int'>\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": "code",
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"collapsed": false,
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|
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