Getting started with Python =========================== This tutorial is based on material from the `python-for-geopeople `_ -course and `Software Carpentry group's `__ lessons on `Programming with Python `__. For a full documentation of Python, please refer to the `online documentation `_. Throughout the course, remember to ask help from others and to search for documentation and related discussions online (just google it!). Python console --------------- There are different ways of using the Python-interpreter. We will start familiarizing ourselves with the python syntax via the IPython console window in Spyder, which is an integrated development environment (IDE) for scientific programming with Python, and later we will learn how to define our own functions. 1. Open Spyder from Start> All Programs > Anaconda3 > Spyder .. note:: For now, in the Kumpula GIS-labs, you have to start Spyder by navigating to the folder ``C:\HYapp\Anaconda3\Scripts`` and double clicking ``spyder.exe``. .. figure:: img/Spyder.png :alt: Spyder IDE Spyder IDE The console-window is in the bottom-right corner of the Spyder-window. For now, you can ignore the other components in Spyder, we will get back to those a bit later. Variables and basic arithmetic ------------------------------ We will start our Python lesson by learning a bit of the basic operations you can perform using Python. 1. Python can be used as a simple calculator. .. code:: python >>> 1 + 1 2 >>> 5 * 7 35 2. You can use Python for more advanced math by using *functions*. Functions are pieces of code that perform a single action such as printing information to the screen (e.g., the ``print()`` function). Functions exist for a huge number of operations in Python. .. code:: python >>> sin(3) Traceback (most recent call last): File "", line 1, in NameError: name 'sin' is not defined >>> sqrt(4) Traceback (most recent call last): File "", line 1, in NameError: name 'sqrt' is not defined Wait, what? Python can't calculate square roots or do basic trigonometry? Of course it can, but we need one more step. 3. The list of basic arithmetic operations that can be done by default in Python is in the table below. +----------------+--------+---------------+----------------+ | Operation | Symbol | Example syntax| Returned value | +================+========+===============+================+ | Addition | ``+`` | ``2 + 2`` | ``4`` | +----------------+--------+---------------+----------------+ | Subtraction | ``-`` | ``4 - 2`` | ``2`` | +----------------+--------+---------------+----------------+ | Multiplication | ``*`` | ``2 * 3`` | ``6`` | +----------------+--------+---------------+----------------+ | Division | ``/`` | ``4 / 2`` | ``2`` | +----------------+--------+---------------+----------------+ |Exponentiation | ``**`` | ``2**3`` | ``8`` | +----------------+--------+---------------+----------------+ For anything more advanced, we need to load a *library*. libraries --------- .. code:: python >>> import math >>> math.sin(3) 0.1411200080598672 >>> math.sqrt(4) 2.0 A *library* is a group of code items such as functions that are related to one another. Libraries are loaded using ``import``. Functions that are part of the library ``libraryname`` could then be used by typing ``libraryname.functionname()``. For example, ``sin()`` is a function that is part of the ``math`` library, and used by typing ``math.sin()`` with some number between the parentheses. Libraries may also contain constants such as ``math.pi``. .. code:: python >>> math.pi 3.141592653589793 >>> math.sin(math.pi) 1.2246467991473532e-16 4. Functions can also be combined. .. code:: python >>> print(math.sqrt(4)) 2.0 >>> print('The square root of 4 is',math.sqrt(4)) The square root of 4 is 2.0 5. *Variables* can be used to store values calculated in expressions and used for other calculations. .. code:: python >>> temp_celsius = 10.0 >>> print(temp_celsius) 10.0 >>> print('temperature in Fahrenheit:', 9/5 * temp_celsius + 32) temperature in Fahrenheit: 50.0 Above, we also see one common format for *good* variable naming, separation of words by underscores ``_`` (e.g., ``temp_celsius``). This is called pothole\_case\_naming. We'll see another below. 6. Values stored in *variables* can also be updated. .. code:: python >>> temp_celsius = 15.0 >>> print('temperature in Celsius is now:', temp_celsius) temperature in Celsius is now: 15.0 >>> TemperatureInFahrenheit = 9/5 * temp_celsius + 32 >>> print('temperature in Celsius:', temp_celsius, 'and in Fahrenheit:', TemperatureInFahrenheit) temperature in Celsius: 15.0 and in Fahrenheit: 59.0 An alternative to naming variables using pothole\_case\_naming is to use capital letters for each word with no spaces between (e.g., ``TemperatureInFahrenheit``). This is called CamelCaseNaming. Both options are easy to read and help you use *good* variable names. After all, *people* should be able to easily understand what different variables contain. 7. Note that changing the values of a variable does not affect those of other variables. .. code:: python >>> temp_celsius = 20.0 >>> print('temperature in Celsius is now:', temp_celsius, 'and temperature in Fahrenheit is still:', TemperatureInFahrenheit) temperature in Celsius is now: 20.0 and temperature in Fahrenheit is still: 59.0 8. One of the nice options in IPython is that you can see which variables are in memory by typing ``%whos``. .. code:: python >>> %whos Variable Type Data/Info --------------------------------------------- TemperatureInFahrenheit float 59.0 temp_celsius float 20.0 9. There are 4 basic *data types* in Python as shown in the table below. +----------------+----------------------+-----------------+ | Data type name | Data type | Example | +================+======================+=================+ | `int` | Whole integer values | `4` | +----------------+----------------------+-----------------+ | `float` | Decimal values | `3.1415` | +----------------+----------------------+-----------------+ | `str` | Character strings | `'Hot'` | +----------------+----------------------+-----------------+ | `bool` | True/false values | `True` | +----------------+----------------------+-----------------+ The data types are displayed when using ``%whos``, but can also be found using the``type()`` function. As you will see, the data types are important because some are not compatible with one another. .. code:: python >>> WeatherForecast = 'Hot' >>> type(WeatherForecast) str >>> type(TemperatureInFahrenheit) float >>> TemperatureInFahrenheit = TemperatureInFahrenheit + 5.0 * WeatherForecast --------------------------------------------------------------------------- TypeError Traceback (most recent call last) in () ----> 1 TemperatureInFahrenheit = TemperatureInFahrenheit + 5.0 * WeatherForecast TypeError: can't multiply sequence by non-int of type 'float' Summary --------- In this first section we have seen a bit of what we can do in Python: defining variables, basic arithmetic, importing libraries, using functions, and combining these things to put the computer to work for us.