0_3_python_functions_and_classes.ipynb

{"nbformat":4,"nbformat_minor":0,"metadata":{"kernelspec":{"display_name":"Python 3","language":"python","name":"python3"},"language_info":{"codemirror_mode":{"name":"ipython","version":3},"file_extension":".py","mimetype":"text/x-python","name":"python","nbconvert_exporter":"python","pygments_lexer":"ipython3","version":"3.7.3"},"colab":{"name":"0_3_python_functions_and_classes.ipynb","provenance":[],"collapsed_sections":["LPD5GTMdPwKf","MviT5HxnPwLO"]}},"cells":[{"cell_type":"markdown","metadata":{"id":"P8IaUkQSPwKd"},"source":["Original notebook by Sem Vijverberg for the series 'Python skills for research in Hydrology' for the master Hydrology at the Vrije Universiteit Amsterdam (2019)."]},{"cell_type":"markdown","metadata":{"id":"s5r0i3FCPwKe"},"source":["# Functions and Classes"]},{"cell_type":"markdown","metadata":{"id":"LPD5GTMdPwKf"},"source":["## Functions\n","Functions are essenstial for writing clear and reproducable code. It allows you to re-use a part of code."]},{"cell_type":"code","metadata":{"id":"BBkH9Y-_PwKg"},"source":["# let's define a simple function\n","def x_squared(x):\n","    y = x**2\n","    return y"],"execution_count":null,"outputs":[]},{"cell_type":"code","metadata":{"id":"DwIr4sWqPwKm","colab":{"base_uri":"https://localhost:8080/"},"executionInfo":{"status":"ok","timestamp":1604057042077,"user_tz":-60,"elapsed":765,"user":{"displayName":"Sem Vijverberg","photoUrl":"","userId":"13865882758081973772"}},"outputId":"0c34079b-b32f-498d-c737-69d5b1c1ae42"},"source":["# let's plug in a value for x\n","x = 2\n","y = x_squared(x)\n","print(y)\n","# or\n","print(x_squared(2))"],"execution_count":null,"outputs":[{"output_type":"stream","text":["4\n","4\n"],"name":"stdout"}]},{"cell_type":"markdown","metadata":{"id":"x6aPLt0jPwKs"},"source":["Within functions you are dealing with _local_ variables, while outside of functions you are dealing with _global_ variables. \n"," "]},{"cell_type":"code","metadata":{"id":"pPfuRCKqPwKs","colab":{"base_uri":"https://localhost:8080/"},"executionInfo":{"status":"ok","timestamp":1604057042428,"user_tz":-60,"elapsed":514,"user":{"displayName":"Sem Vijverberg","photoUrl":"","userId":"13865882758081973772"}},"outputId":"ad8e939f-7153-41eb-a582-67b4bd220c39"},"source":["# A global variable can be accesed from your memory everywhere outside a function\n","# x was defined earlier and is in my memory\n","print(x)"],"execution_count":null,"outputs":[{"output_type":"stream","text":["2\n"],"name":"stdout"}]},{"cell_type":"markdown","metadata":{"id":"-SshbcO6PwK4"},"source":["_local_ variables are only defined within a function. For example, let's see what happens if I define the variable _z_ inside the original function."]},{"cell_type":"code","metadata":{"id":"xIlJORkBPwK4","colab":{"base_uri":"https://localhost:8080/","height":215},"executionInfo":{"status":"error","timestamp":1604057043401,"user_tz":-60,"elapsed":708,"user":{"displayName":"Sem Vijverberg","photoUrl":"","userId":"13865882758081973772"}},"outputId":"5d4917b0-0b87-4b6d-851a-32a96f591a1f"},"source":["def x_squared(x):\n","    y = x**2\n","    z = 2*y\n","    return y\n","\n","# let's run the function:\n","y = x_squared(2)\n","print(y)\n","# if I now ask for the value of z, it will give a NameError. NameErrors are given when a variable is not defined. \n","print(z)"],"execution_count":null,"outputs":[{"output_type":"stream","text":["4\n"],"name":"stdout"},{"output_type":"error","ename":"NameError","evalue":"ignored","traceback":["\u001b[0;31m---------------------------------------------------------------------------\u001b[0m","\u001b[0;31mNameError\u001b[0m                                 Traceback (most recent call last)","\u001b[0;32m<ipython-input-4-fad898af6f19>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[1;32m      8\u001b[0m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0my\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m      9\u001b[0m \u001b[0;31m# if I now ask for the value of z, it will give a NameError. NameErrors are given when a variable is not defined.\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m---> 10\u001b[0;31m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mz\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m","\u001b[0;31mNameError\u001b[0m: name 'z' is not defined"]}]},{"cell_type":"markdown","metadata":{"id":"cq6G3p6JPwK8"},"source":["Since we didn't return z from the function, it stays within the function as a _local_ variable. So let's also return z:"]},{"cell_type":"code","metadata":{"id":"U9sDR1ddPwK8","colab":{"base_uri":"https://localhost:8080/"},"executionInfo":{"status":"ok","timestamp":1604057055425,"user_tz":-60,"elapsed":588,"user":{"displayName":"Sem Vijverberg","photoUrl":"","userId":"13865882758081973772"}},"outputId":"449ac8dd-034a-43d9-9a39-f167477ad775"},"source":["def x_squared(x):\n","    y = x**2\n","    z = 2*y\n","    return y, z\n","\n","# let's run the function:\n","y, z = x_squared(2)\n","print(y)\n","print(z)"],"execution_count":null,"outputs":[{"output_type":"stream","text":["4\n","8\n"],"name":"stdout"}]},{"cell_type":"markdown","metadata":{"id":"MuZA3ouEPwLA"},"source":["## positional versus optional arguments\n","Also called non-default vs. default arguments"]},{"cell_type":"markdown","metadata":{"id":"zRHclm_FPwLB"},"source":["Optional arguments can be defined when you 'define' the function, see example below. If you don't specify _i_, it will use the default value 3. This is efficient because it allows to write flexible functions which work 90% of the time, because 90% of the time you need the default setting. But when you ever need to be a bit flexible, you can adapt. "]},{"cell_type":"code","metadata":{"id":"k3DdTP7VPwLB","colab":{"base_uri":"https://localhost:8080/"},"executionInfo":{"status":"ok","timestamp":1636453629708,"user_tz":-60,"elapsed":653,"user":{"displayName":"Sem Vijverberg","photoUrl":"https://lh3.googleusercontent.com/a-/AOh14GgP3ekzXBWjWxtQi_zniAyCcTS_7t_IAIO3tTUdRw=s64","userId":"13865882758081973772"}},"outputId":"bed9961b-a9f6-4105-9fcb-401fbed1c6ea"},"source":["def x_squared(x, i=3):\n","    y = x**2\n","    z = 2*i\n","    return y, z\n","# let's run the function:\n","y, z = x_squared(2)\n","print(y)\n","print(z)"],"execution_count":null,"outputs":[{"output_type":"stream","name":"stdout","text":["4\n","6\n"]}]},{"cell_type":"markdown","metadata":{"id":"aP8qhpACOefZ"},"source":["Arguments without a default value are often called positional arguments, since the order (position) of the argument determines the value the arguments will get."]},{"cell_type":"code","metadata":{"colab":{"base_uri":"https://localhost:8080/"},"id":"KlxL9BeFOhZW","executionInfo":{"status":"ok","timestamp":1636453631359,"user_tz":-60,"elapsed":8,"user":{"displayName":"Sem Vijverberg","photoUrl":"https://lh3.googleusercontent.com/a-/AOh14GgP3ekzXBWjWxtQi_zniAyCcTS_7t_IAIO3tTUdRw=s64","userId":"13865882758081973772"}},"outputId":"7c4dadf3-ed9b-4dac-93fc-071e9f251c4c"},"source":["def x_squared(x, i):\n","    y = x**2\n","    z = 2*i\n","    return y, z\n","# let's run the function:\n","print(x_squared(2, 3))\n","print(x_squared(3, 2))"],"execution_count":null,"outputs":[{"output_type":"stream","name":"stdout","text":["(4, 6)\n","(9, 4)\n"]}]},{"cell_type":"markdown","metadata":{"id":"MviT5HxnPwLO"},"source":["## kwargs and args\n","\n","kwargs are keyword arguments, args are simply arguments in a list. \n","\n","kwargs are basically dictionaries with variables as keys and their concomitant values. To use a normal dictionary as kwrgs, use the **kwrgs: For example: "]},{"cell_type":"code","metadata":{"id":"_7SIXeG8PwLO","outputId":"ddc7023a-7683-4bee-c169-a0481eb63d64"},"source":["kwrgs = {'x' : 2,\n","         'i' : 4}\n","def x_squared(x, i=3):\n","    y = x**2\n","    z = 2*i\n","    return y, z\n","# let's run the function:\n","y, z = x_squared(**kwrgs)\n","print(y)\n","print(z)"],"execution_count":null,"outputs":[{"output_type":"stream","text":["4\n","8\n"],"name":"stdout"}]},{"cell_type":"markdown","metadata":{"id":"8tIV9DxRPwLT"},"source":["args are given in list form (in order of function variables), for args you need to add one * instead of two **:"]},{"cell_type":"code","metadata":{"id":"JWo7GvyJPwLT","outputId":"bc223d4b-02a9-49f9-c487-934f84a47f79"},"source":["args = [2, 4]\n","def x_squared(x, i=3):\n","    y = x**2\n","    z = 2*i\n","    return y, z\n","# let's run the function:\n","y, z = x_squared(*args)\n","print(y)\n","print(z)"],"execution_count":null,"outputs":[{"output_type":"stream","text":["4\n","8\n"],"name":"stdout"}]},{"cell_type":"markdown","metadata":{"id":"Flb_cW2rPwLe"},"source":["If you don't have any default value, you can make you function more clear by stating a certain data type input. For example, perhaps you want the above function to only except floats (will also except integers). "]},{"cell_type":"code","metadata":{"id":"yJQBJkYxPwLh","outputId":"5c4d8f81-1e98-4eb8-bacc-f8e8b2fc906e"},"source":["args = [2., 4]\n","def x_squared(x=float, i=float):\n","    y = x**2\n","    z = 2*i\n","    return y, z\n","# let's run the function:\n","y, z = x_squared(*args)\n","print(y)\n","print(z)\n","\n"],"execution_count":null,"outputs":[{"output_type":"stream","text":["4.0\n","8\n"],"name":"stdout"}]},{"cell_type":"code","metadata":{"id":"urFgbSZ9PwLn","outputId":"74a82741-1a75-4e1f-c25c-310834e43876"},"source":["# Indeed, if you now supply a list of values to x..\n","\n","kwrgs = {'x' : [0,1,2],\n","         'i' : 4}\n","def x_squared(x=float, i=float):\n","    y = x**2\n","    z = 2*i\n","    return y, z\n","# let's run the function:\n","y, z = x_squared(**kwrgs)\n","print(y)\n","print(z)\n"],"execution_count":null,"outputs":[{"output_type":"error","ename":"TypeError","evalue":"unsupported operand type(s) for ** or pow(): 'list' and 'int'","traceback":["\u001b[0;31m---------------------------------------------------------------------------\u001b[0m","\u001b[0;31mTypeError\u001b[0m                                 Traceback (most recent call last)","\u001b[0;32m<ipython-input-58-a079507b8a56>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m\u001b[0m\n\u001b[1;32m      8\u001b[0m     \u001b[0;32mreturn\u001b[0m \u001b[0my\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mz\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m      9\u001b[0m \u001b[0;31m# let's run the function:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m---> 10\u001b[0;31m \u001b[0my\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mz\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0mx_squared\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m**\u001b[0m\u001b[0mkwrgs\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m     11\u001b[0m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0my\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m     12\u001b[0m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mz\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n","\u001b[0;32m<ipython-input-58-a079507b8a56>\u001b[0m in \u001b[0;36mx_squared\u001b[0;34m(x, i)\u001b[0m\n\u001b[1;32m      4\u001b[0m          'i' : 4}\n\u001b[1;32m      5\u001b[0m \u001b[0;32mdef\u001b[0m \u001b[0mx_squared\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mx\u001b[0m\u001b[0;34m=\u001b[0m\u001b[0mfloat\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mi\u001b[0m\u001b[0;34m=\u001b[0m\u001b[0mfloat\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m:\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 6\u001b[0;31m     \u001b[0my\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0mx\u001b[0m\u001b[0;34m**\u001b[0m\u001b[0;36m2\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m\u001b[1;32m      7\u001b[0m     \u001b[0mz\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0;36m2\u001b[0m\u001b[0;34m*\u001b[0m\u001b[0mi\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m      8\u001b[0m     \u001b[0;32mreturn\u001b[0m \u001b[0my\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mz\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n","\u001b[0;31mTypeError\u001b[0m: unsupported operand type(s) for ** or pow(): 'list' and 'int'"]}]},{"cell_type":"code","metadata":{"id":"LqM5k00nPwLq"},"source":[""],"execution_count":null,"outputs":[]}]}