mirror of
https://github.com/rasbt/python_reference.git
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mutable objects as default arguments
This commit is contained in:
rasbt
parent
38092ac583
commit
c177a57002
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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:faa74a34746bf250ef2d72e308074083ee5e60789203d70f630f8c67a709e6fe"
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"signature": "sha256:06e485535e22c756262f09d4477108b57d0fa1029f9831ee77a177267f5afc7f"
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"nbformat": 3,
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"nbformat_minor": 0,
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@ -46,7 +46,8 @@
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"- [Don't use mutable objects as default arguments for functions!](#def_mutable_func)\n",
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"- [Be aware of the consuming generator](#consuming_generator)\n",
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"- [`bool` is a subclass of `int`](#bool_int)\n",
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"- [About lambda and closures-in-a-loop pitfall](#lambda_closure)"
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"- [About lambda and closures-in-a-loop pitfall](#lambda_closure)\n",
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"- [Python's LEGB scope resolution and the keywords `global` and `nonlocal`](#python_legb)"
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]
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},
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{
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@ -594,7 +595,9 @@
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"\n",
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"## About lambda and closures-in-a-loop pitfall\n",
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"\n",
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"The following example illustrates how the (last) `lambda` is being reused:"
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"The following example illustrates how the (last) `lambda` is being reused:\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))"
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]
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},
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{
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@ -654,13 +657,167 @@
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],
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"prompt_number": 25
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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_legb'></a>\n",
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"\n",
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"## Python's LEGB scope resolution and the keywords `global` and `nonlocal`"
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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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"There is nothing particularly surprising about Python's LEGB scope resolution (Local -> Enclosed -> Global -> Built-in), but it is still useful to take a look at some examples!"
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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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"### `global` vs. `local`\n",
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"\n",
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"According to the LEGB rule, Python will first look for a variable in the local scope. So if we set the variable `x = 1` in the `local`ly in the function's scope, it won't have an effect on the `global` `x`."
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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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"input": [
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"x = 0\n",
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"def in_func():\n",
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" x = 1\n",
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" print('in_func:', x)\n",
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" \n",
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"in_func()\n",
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"print('global:', x)"
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],
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"language": "python",
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"metadata": {},
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"outputs": []
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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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"in_func: 1\n",
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"global: 0\n"
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]
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}
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],
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"prompt_number": 33
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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 want to modify the `global` x via a function, we can simply use the `global` keyword to import the variable into the function's scope:"
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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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"x = 0\n",
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"def in_func():\n",
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" global x\n",
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" x = 1\n",
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" print('in_func:', x)\n",
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" \n",
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"in_func()\n",
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"print('global:', x)"
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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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"in_func: 1\n",
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"global: 1\n"
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]
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}
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],
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"prompt_number": 34
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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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"### `local` vs. `enclosed`\n",
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"\n",
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"Now, let us take a look at `local` vs. `enclosed`. Here, we set the variable `x = 1` in the `outer` function and set `x = 1` in the enclosed function `inner`. Since `inner` looks in the local scope first, it won't modify `outer`'s `x`."
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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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"def outer():\n",
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" x = 1\n",
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" print('outer before:', x)\n",
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" def inner():\n",
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" x = 2\n",
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" print(\"inner:\", x)\n",
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" inner()\n",
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" print(\"outer after:\", x)\n",
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"outer()"
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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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"outer before: 1\n",
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"inner: 2\n",
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"outer after: 1\n"
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]
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}
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],
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"prompt_number": 36
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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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"Here is where the `nonlocal` keyword comes in handy - it allows us to modify the `x` variable in the `enclosed` scope:"
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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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"def outer():\n",
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" x = 1\n",
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" print('outer before:', x)\n",
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" def inner():\n",
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" nonlocal x\n",
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" x = 2\n",
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" print(\"inner:\", x)\n",
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" inner()\n",
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" print(\"outer after:\", x)\n",
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"outer()"
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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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"outer before: 1\n",
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"inner: 2\n",
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"outer after: 2\n"
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]
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}
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],
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"prompt_number": 35
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}
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],
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"metadata": {}
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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:faa74a34746bf250ef2d72e308074083ee5e60789203d70f630f8c67a709e6fe"
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"signature": "sha256:06e485535e22c756262f09d4477108b57d0fa1029f9831ee77a177267f5afc7f"
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},
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"nbformat": 3,
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"nbformat_minor": 0,
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|
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@ -46,7 +46,8 @@
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"- [Don't use mutable objects as default arguments for functions!](#def_mutable_func)\n",
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"- [Be aware of the consuming generator](#consuming_generator)\n",
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"- [`bool` is a subclass of `int`](#bool_int)\n",
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"- [About lambda and closures-in-a-loop pitfall](#lambda_closure)"
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"- [About lambda and closures-in-a-loop pitfall](#lambda_closure)\n",
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"- [Python's LEGB scope resolution and the keywords `global` and `nonlocal`](#python_legb)"
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]
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},
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{
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@ -594,7 +595,9 @@
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"\n",
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"## About lambda and closures-in-a-loop pitfall\n",
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"\n",
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"The following example illustrates how the (last) `lambda` is being reused:"
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"The following example illustrates how the (last) `lambda` is being reused:\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))"
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]
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},
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{
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@ -654,13 +657,167 @@
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],
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"prompt_number": 25
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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_legb'></a>\n",
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"\n",
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"## Python's LEGB scope resolution and the keywords `global` and `nonlocal`"
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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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"There is nothing particularly surprising about Python's LEGB scope resolution (Local -> Enclosed -> Global -> Built-in), but it is still useful to take a look at some examples!"
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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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"### `global` vs. `local`\n",
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"\n",
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"According to the LEGB rule, Python will first look for a variable in the local scope. So if we set the variable `x = 1` in the `local`ly in the function's scope, it won't have an effect on the `global` `x`."
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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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"input": [
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"x = 0\n",
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"def in_func():\n",
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" x = 1\n",
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" print('in_func:', x)\n",
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" \n",
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"in_func()\n",
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"print('global:', x)"
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],
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"language": "python",
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"metadata": {},
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"outputs": []
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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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"in_func: 1\n",
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"global: 0\n"
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]
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}
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],
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"prompt_number": 33
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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 want to modify the `global` x via a function, we can simply use the `global` keyword to import the variable into the function's scope:"
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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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"x = 0\n",
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"def in_func():\n",
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" global x\n",
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" x = 1\n",
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" print('in_func:', x)\n",
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" \n",
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"in_func()\n",
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"print('global:', x)"
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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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"in_func: 1\n",
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"global: 1\n"
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]
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}
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],
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"prompt_number": 34
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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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"### `local` vs. `enclosed`\n",
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"\n",
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"Now, let us take a look at `local` vs. `enclosed`. Here, we set the variable `x = 1` in the `outer` function and set `x = 1` in the enclosed function `inner`. Since `inner` looks in the local scope first, it won't modify `outer`'s `x`."
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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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"def outer():\n",
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" x = 1\n",
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" print('outer before:', x)\n",
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" def inner():\n",
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" x = 2\n",
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" print(\"inner:\", x)\n",
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" inner()\n",
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" print(\"outer after:\", x)\n",
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"outer()"
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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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"outer before: 1\n",
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"inner: 2\n",
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"outer after: 1\n"
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]
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}
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],
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"prompt_number": 36
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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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"Here is where the `nonlocal` keyword comes in handy - it allows us to modify the `x` variable in the `enclosed` scope:"
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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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"def outer():\n",
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" x = 1\n",
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" print('outer before:', x)\n",
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" def inner():\n",
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" nonlocal x\n",
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" x = 2\n",
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" print(\"inner:\", x)\n",
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" inner()\n",
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" print(\"outer after:\", x)\n",
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"outer()"
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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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"outer before: 1\n",
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"inner: 2\n",
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"outer after: 2\n"
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]
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}
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],
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"prompt_number": 35
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}
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],
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"metadata": {}
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|
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