Day1-Getting-started-with-python.rst

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.

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.

   >>> 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.

   >>> sin(3)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   NameError: name 'sin' is not defined
   >>> sqrt(4)
   Traceback (most recent call last):
     File "<stdin>", line 1, in <module>
   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

>>> 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.

>>> math.pi
3.141592653589793
>>> math.sin(math.pi)
1.2246467991473532e-16

4. Functions can also be combined.

   >>> 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.

   >>> 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.

   >>> 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.

   >>> 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.

   >>> %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.

>>> WeatherForecast = 'Hot'
>>> type(WeatherForecast)
str
>>> type(TemperatureInFahrenheit)
float
>>> TemperatureInFahrenheit = TemperatureInFahrenheit + 5.0 * WeatherForecast
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-21-7046bdc97a54> in <module>()
----> 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.