Basic elements of Python

Sources

This lesson is inspired by the Programming in Python lessons from the Software Carpentry organization.

Getting started

1. We can start by opening a new IPython window. To do this, open a new Terminal window and type ipython. You should see an IPython prompt that looks something like the following:

   $ ipython
   Python 3.5.2 |Anaconda custom (x86_64)| (default, Jul  2 2016, 17:52:12)
   Type "copyright", "credits" or "license" for more information.
   
   IPython 5.1.0 -- An enhanced Interactive Python.
   ?         -> Introduction and overview of IPython's features.
   %quickref -> Quick reference.
   help      -> Python's own help system.
   object?   -> Details about 'object', use 'object??' for extra details.
   
   In [1]:
   

Now we are ready to start.

Variables, arithmetic and modules

We are assuming you already have some familiarity with Python, but we’ll quickly review some basic features and operations.

1. Python can be used as a simple calculator.

   >>> 1 + 1
   2
   >>> 5 * 7
   35
   

2. 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 use a function. 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. Python has a few basic functions available by default, but to use most functions we need to load a load a module.

   >>> import math
   >>> math.sin(3)
   0.1411200080598672
   >>> math.sqrt(4)
   2.0
   

   A module (or library) is a group of code items such as functions that are related to one another. Modules are loaded using import. Functions that are part of the module modulename could then be used by typing modulename.functionname(). For example, sin() is a function that is part of the math module, and used by typing math.sin() with some number between the parentheses. Modules 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.

   >>> tempCelsius = 10.0
   >>> print(tempCelsius)
   10.0
   >>> print('temperature in Fahrenheit:', 9/5 * tempCelsius + 32)
   temperature in Fahrenheit: 50.0
   

6. Values stored in variables can also be updated.

   >>> tempCelsius = 15.0
   >>> print('temperature in Celsius is now:', tempCelsius)
   temperature in Celsius is now: 15.0
   >>> tempFahrenheit = 9/5 * tempCelsius + 32
   >>> print('temperature in Celsius:', tempCelsius, 'and in Fahrenheit:', tempFahrenheit)
   temperature in Celsius: 15.0 and in Fahrenheit: 59.0
   

7. 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
   

8. 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'
   

Lists and indices

Let’s now consider an example of some rock samples collected during a recent field excursion. Rather than having individual variables for each of the samples, we can store many related values in a collection. The simplest type of collection in Python is a list.

1. Let’s first create a list of selected SampleID values.

   >>> SampleIDs = ['DW-NP-03', 'DW-NP-12', 'DW-NP-33', 'DW-NP-48']
   >>> print(SampleIDs)
   ['DW-NP-03', 'DW-NP-12', 'DW-NP-33', 'DW-NP-48']
   >>> type(SampleIDs)
   list
   

   Here we have a list of 4 SampleID values in a list called SampleIDs. As you can see, the type() function recognizes this as a list. Lists can be created using the square brackets ([ and ]), with commas separating the values in the list.

2. To access an individual value in the list we need to use an index value. An index value is a number that refers to a given position in the list. Let’s check out the first value in our list as an example:

   >>> print(SampleIDs[1])
   'DW-NP-12'
   

   Remember, Python (and many other programming languages) start values stored in collections with the index value 0.

3. We can find the length of a list using the len() function.

   >>> len(SampleIDs)
   4
   

   Just as expected, there are 4 values in our list and len(SampleIDs) returns a value of 4.

Exercise - Getting to know lists

• Create a Python list containing 4 items/values and store it with the variable mylist.
• After you have created the list, check its data type, then use the index values to check the data types of the contents of the list.
• What kinds of data can you store in lists? Does it all need to be the same data type?