Here we look at some of the most basic aspects of how Python handles items of data.
To start, make sure you are in the class environment, by opening a console window and typing
conda activate g420
at the prompt. Then start up the Python interpreter within
python
You should see something like
Python 3.9.7 | packaged by conda-forge | (default, Sep 29 2021, 19:20:46)
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>
From here on, type python commands at the >>> prompt. When we’re done, you can close the interpreter by typing exit() at the prompt, and then close the environment with conda deactivate.
It’s traditional when learning to program to first learn how to get the computer to ‘say’ “Hello world”. Here’s what that program looks like in Python:
>>> print("Hello World")
Hello World
Type that at the python prompt and see what happens.
Try running the commands below and see what happens.
>>> 40 + 2
>>> 43 - 1
>>> 6 * 7
>>> 84 / 2
So… we can use Python as a not-so-simple calculator! Hurrah! Just what the world needs, another calculator. One of the other basic arithmetic operators is ** which is used for ‘raise to the power of’
>>> # Close enough
>>> 6.48 ** 2
>>> # What about this
>>> 42 ** 0.5
In the two lines above I’ve used comments which denoted by the # symbol. The Python interpreter ignores anything on the line after a # symbol. Working interactively in IDLE, you can simply ignore these—they are there for your benefit as a human, and are irrelevant to the Python interpreter. This might seem pointless, but it is really useful to add comments to working code (i.e. scripts and programs) so that other human readers (most likely you 6 months later when you come back to the code) can understand what it is supposed to do.
Every item of data in Python has a value and a type. The value is the content of the data, like the numeric content 42 or the text content Hello World!. But these two values are of different types, one is a number, the other is some text. Python has some specific types which it is important to know about.
>>> type(42)
>>> type(42.0)
>>> type('42')
Here three pieces of data which humans might be inclined to consider identical are actually considered different because they are different types, int, float, and str.
int is an integer, i.e., a numeric value with no decimal partfloat is a floating point number, i.e., a numeric value that has a decimal partstr is a string denoting a sequence of charactersStrings are distinguished from numeric types by using quote marks. It is worth noting that we can use either single '' or double "" quote marks. As far as Python is concerned 'spam' and "spam" are the same value:
>>> "spam" == 'spam'
We’ll get to == in a bit. The fact that we can mix quote marks means that we can include them in a string, which is useful, so for example we can do
>>> print("'Hello, World!'")
How is this different from the first example of hello world at the start of these instructions?
Numbers are manipulated using arithmetic operators such as +, -, * and so on, generally these don’t make much sense for strings, although + and * still work.
>>> 3 * 'spam'
>>> 'spam' + 'ham'