(ProgWerk) Python Fundamentals
Last updated on 2024-03-10 | Edit this page
Estimated time: 30 minutes
Overview
Questions
- What basic data types can I work with in Python?
- How can I create a new variable in Python?
- How do I use a function?
- Can I change the value associated with a variable after I create it?
Objectives
- Assign values to variables.
Programmier-Werkstatt 2024
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Variables
Any Python interpreter can be used as a calculator:
OUTPUT
23
This is great but not very interesting. To do anything useful with
data, we need to assign its value to a variable. In Python, we
can assign a value to a variable, using the equals sign
=
. For example, we can track the weight of a patient who
weighs 60 kilograms by assigning the value 60
to a variable
weight_kg
:
From now on, whenever we use weight_kg
, Python will
substitute the value we assigned to it. In layperson’s terms, a
variable is a name for a value.
In Python, variable names:
- can include letters, digits, and underscores
- cannot start with a digit
- are case sensitive.
This means that, for example:
-
weight0
is a valid variable name, whereas0weight
is not -
weight
andWeight
are different variables
Types of data
Python knows various types of data. Three common ones are:
- integer numbers
- floating point numbers, and
- strings.
In the example above, variable weight_kg
has an integer
value of 60
. If we want to more precisely track the weight
of our patient, we can use a floating point value by executing:
To create a string, we add single or double quotes around some text. To identify and track a patient throughout our study, we can assign each person a unique identifier by storing it in a string:
Using Variables in Python
Once we have data stored with variable names, we can make use of it in calculations. We may want to store our patient’s weight in pounds as well as kilograms:
We might decide to add a prefix to our patient identifier:
Built-in Python functions
To carry out common tasks with data and variables in Python, the
language provides us with several built-in functions. To display information to
the screen, we use the print
function:
OUTPUT
132.66
inflam_001
When we want to make use of a function, referred to as calling the
function, we follow its name by parentheses. The parentheses are
important: if you leave them off, the function doesn’t actually run!
Sometimes you will include values or variables inside the parentheses
for the function to use. In the case of print
, we use the
parentheses to tell the function what value we want to display. We will
learn more about how functions work and how to create our own in later
episodes.
We can display multiple things at once using only one
print
call:
OUTPUT
inflam_001 weight in kilograms: 60.3
We can also call a function inside of another function call. For example,
Python has a built-in function called type
that tells you a
value’s data type:
OUTPUT
<class 'float'>
<class 'str'>
Moreover, we can do arithmetic with variables right inside the
print
function:
OUTPUT
weight in pounds: 132.66
The above command, however, did not change the value of
weight_kg
:
OUTPUT
60.3
To change the value of the weight_kg
variable, we have
to assign weight_kg
a new value using the
equals =
sign:
OUTPUT
weight in kilograms is now: 65.0
Variables as Sticky Notes
A variable in Python is analogous to a sticky note with a name written on it: assigning a value to a variable is like putting that sticky note on a particular value.
Using this analogy, we can investigate how assigning a value to one variable does not change values of other, seemingly related, variables. For example, let’s store the subject’s weight in pounds in its own variable:
PYTHON
# There are 2.2 pounds per kilogram
weight_lb = 2.2 * weight_kg
print('weight in kilograms:', weight_kg, 'and in pounds:', weight_lb)
OUTPUT
weight in kilograms: 65.0 and in pounds: 143.0
Everything in a line of code following the ‘#’ symbol is a comment that is ignored by Python. Comments allow programmers to leave explanatory notes for other programmers or their future selves.
Similar to above, the expression 2.2 * weight_kg
is
evaluated to 143.0
, and then this value is assigned to the
variable weight_lb
(i.e. the sticky note
weight_lb
is placed on 143.0
). At this point,
each variable is “stuck” to completely distinct and unrelated
values.
Let’s now change weight_kg
:
PYTHON
weight_kg = 100.0
print('weight in kilograms is now:', weight_kg, 'and weight in pounds is still:', weight_lb)
OUTPUT
weight in kilograms is now: 100.0 and weight in pounds is still: 143.0
Since weight_lb
doesn’t “remember” where its value comes
from, it is not updated when we change weight_kg
.
OUTPUT
`mass` holds a value of 47.5, `age` does not exist
`mass` still holds a value of 47.5, `age` holds a value of 122
`mass` now has a value of 95.0, `age`'s value is still 122
`mass` still has a value of 95.0, `age` now holds 102
OUTPUT
Hopper Grace
Key Points
- Basic data types in Python include integers, strings, and floating-point numbers.
- Use
variable = value
to assign a value to a variable in order to record it in memory. - Variables are created on demand whenever a value is assigned to them.
- Use
print(something)
to display the value ofsomething
. - Use
# some kind of explanation
to add comments to programs. - Built-in functions are always available to use.