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added args and kwargs
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‎03-Methods and Functions/.ipynb_checkpoints/07-args and kwargs-checkpoint.ipynb

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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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"# `*args` and `**kwargs`\n",
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"\n",
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"Work with Python long enough, and eventually you will encounter `*args` and `**kwargs`. These strange terms show up as parameters in function definitions. What do they do? Let's review a simple function:"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 1,
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"metadata": {},
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"outputs": [
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{
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"data": {
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"text/plain": [
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"5.0"
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]
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},
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"execution_count": 1,
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"metadata": {},
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"output_type": "execute_result"
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}
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],
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"source": [
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"def myfunc(a,b):\n",
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" return sum((a,b))*.05\n",
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"\n",
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"myfunc(40,60)"
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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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"This function returns 5% of the sum of **a** and **b**. In this example, **a** and **b** are *positional* arguments; that is, 40 is assigned to **a** because it is the first argument, and 60 to **b**. Notice also that to work with multiple positional arguments in the `sum()` function we had to pass them in as a tuple.\n",
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"\n",
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"What if we want to work with more than two numbers? One way would be to assign a *lot* of parameters, and give each one a default value."
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]
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},
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{
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"cell_type": "code",
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"execution_count": 2,
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"metadata": {},
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"outputs": [
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{
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"data": {
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"text/plain": [
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"6.0"
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]
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},
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"execution_count": 2,
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"metadata": {},
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"output_type": "execute_result"
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}
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],
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"source": [
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"def myfunc(a=0,b=0,c=0,d=0,e=0):\n",
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" return sum((a,b,c,d,e))*.05\n",
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"\n",
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"myfunc(40,60,20)"
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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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"Obviously this is not a very efficient solution, and that's where `*args` comes in.\n",
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"\n",
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"## `*args`\n",
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"\n",
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"When a function parameter starts with an asterisk, it allows for an *arbitrary number* of arguments, and the function takes them in as a tuple of values. Rewriting the above function:"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 3,
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"metadata": {},
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"outputs": [
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{
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"data": {
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"text/plain": [
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"6.0"
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]
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},
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"execution_count": 3,
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"metadata": {},
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"output_type": "execute_result"
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}
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],
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"source": [
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"def myfunc(*args):\n",
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" return sum(args)*.05\n",
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"\n",
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"myfunc(40,60,20)"
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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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"Notice how passing the keyword \"args\" into the `sum()` function did the same thing as a tuple of arguments.\n",
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"\n",
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"It is worth noting that the word \"args\" is itself arbitrary - any word will do so long as it's preceded by an asterisk. To demonstrate this:"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 4,
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"metadata": {},
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"outputs": [
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{
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"data": {
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"text/plain": [
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"6.0"
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]
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},
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"execution_count": 4,
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"metadata": {},
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"output_type": "execute_result"
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}
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],
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"source": [
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"def myfunc(*spam):\n",
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" return sum(spam)*.05\n",
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"\n",
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"myfunc(40,60,20)"
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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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"## `**kwargs`\n",
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"\n",
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"Similarly, Python offers a way to handle arbitrary numbers of *keyworded* arguments. Instead of creating a tuple of values, `**kwargs` builds a dictionary of key/value pairs. For example:"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 5,
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"metadata": {},
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"outputs": [
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{
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"name": "stdout",
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"output_type": "stream",
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"text": [
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"My favorite fruit is pineapple\n"
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]
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}
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],
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"source": [
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"def myfunc(**kwargs):\n",
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" if 'fruit' in kwargs:\n",
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" print(f\"My favorite fruit is {kwargs['fruit']}\") # review String Formatting and f-strings if this syntax is unfamiliar\n",
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" else:\n",
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" print(\"I don't like fruit\")\n",
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" \n",
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"myfunc(fruit='pineapple')"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 6,
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"metadata": {},
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"outputs": [
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{
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"name": "stdout",
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"output_type": "stream",
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"text": [
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"I don't like fruit\n"
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]
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}
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],
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"source": [
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"myfunc()"
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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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"## `*args` and `**kwargs` combined\n",
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"\n",
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"You can pass `*args` and `**kwargs` into the same function, but `*args` have to appear before `**kwargs`"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 7,
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"metadata": {},
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"outputs": [
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{
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"name": "stdout",
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"output_type": "stream",
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"text": [
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"I like eggs and spam and my favorite fruit is cherries\n",
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"May I have some orange juice?\n"
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]
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}
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],
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"source": [
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"def myfunc(*args, **kwargs):\n",
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" if 'fruit' and 'juice' in kwargs:\n",
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" print(f\"I like {' and '.join(args)} and my favorite fruit is {kwargs['fruit']}\")\n",
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" print(f\"May I have some {kwargs['juice']} juice?\")\n",
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" else:\n",
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" pass\n",
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" \n",
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"myfunc('eggs','spam',fruit='cherries',juice='orange')"
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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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"Placing keyworded arguments ahead of positional arguments raises an exception:"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 8,
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"metadata": {},
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"outputs": [
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{
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"ename": "SyntaxError",
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"evalue": "positional argument follows keyword argument (<ipython-input-8-fc6ff65addcc>, line 1)",
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"output_type": "error",
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"traceback": [
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"\u001b[1;36m File \u001b[1;32m\"<ipython-input-8-fc6ff65addcc>\"\u001b[1;36m, line \u001b[1;32m1\u001b[0m\n\u001b[1;33m myfunc(fruit='cherries',juice='orange','eggs','spam')\u001b[0m\n\u001b[1;37m ^\u001b[0m\n\u001b[1;31mSyntaxError\u001b[0m\u001b[1;31m:\u001b[0m positional argument follows keyword argument\n"
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]
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}
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],
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"source": [
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"myfunc(fruit='cherries',juice='orange','eggs','spam')"
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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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"As with \"args\", you can use any name you'd like for keyworded arguments - \"kwargs\" is just a popular convention.\n",
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"\n",
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"That's it! Now you should understand how `*args` and `**kwargs` provide the flexibilty to work with arbitrary numbers of arguments!"
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]
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}
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],
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"metadata": {
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"kernelspec": {
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"display_name": "Python 3",
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"language": "python",
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"name": "python3"
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},
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"language_info": {
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"codemirror_mode": {
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"name": "ipython",
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"version": 3
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},
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"file_extension": ".py",
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"mimetype": "text/x-python",
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"name": "python",
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"nbconvert_exporter": "python",
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"pygments_lexer": "ipython3",
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"version": "3.6.2"
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}
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},
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"nbformat": 4,
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"nbformat_minor": 2
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}

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