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01-intro.md

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+---
+title: Python Fundamentals
+teaching: 20
+exercises: 10
+---
+
+::::::::::::::::::::::::::::::::::::::: objectives
+
+- Assign values to variables.
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+:::::::::::::::::::::::::::::::::::::::: 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?
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+## Variables
+
+Any Python interpreter can be used as a calculator:
+
+```python
+3 + 5 * 4
+```
+
+```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](../learners/reference.md#assign) a value to a
+[variable](../learners/reference.md#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`:
+
+```python
+weight_kg = 60
+```
+
+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](../learners/reference.md#case-sensitive).
+
+This means that, for example:
+
+- `weight0` is a valid variable name, whereas `0weight` is not
+- `weight` and `Weight` 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:
+
+```python
+weight_kg = 60.3
+```
+
+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:
+
+```python
+patient_id = '001'
+```
+
+## 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:
+
+```python
+weight_lb = 2.2 * weight_kg
+```
+
+We might decide to add a prefix to our patient identifier:
+
+```python
+patient_id = 'inflam_' + patient_id
+```
+
+## Built-in Python functions
+
+To carry out common tasks with data and variables in Python,
+the language provides us with several built-in [functions](../learners/reference.md#function).
+To display information to the screen, we use the `print` function:
+
+```python
+print(weight_lb)
+print(patient_id)
+```
+
+```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:
+
+```python
+print(patient_id, 'weight in kilograms:', weight_kg)
+```
+
+```output
+inflam_001 weight in kilograms: 60.3
+```
+
+We can also call a function inside of another
+[function call](../learners/reference.md#function-call).
+For example, Python has a built-in function called `type` that tells you a value's data type:
+
+```python
+print(type(60.3))
+print(type(patient_id))
+```
+
+```output
+<class 'float'>
+<class 'str'>
+```
+
+Moreover, we can do arithmetic with variables right inside the `print` function:
+
+```python
+print('weight in pounds:', 2.2 * weight_kg)
+```
+
+```output
+weight in pounds: 132.66
+```
+
+The above command, however, did not change the value of `weight_kg`:
+
+```python
+print(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:
+
+```python
+weight_kg = 65.0
+print('weight in kilograms is now:', weight_kg)
+```
+
+```output
+weight in kilograms is now: 65.0
+```
+
+:::::::::::::::::::::::::::::::::::::::::  callout
+
+## 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.
+
+![](fig/python-sticky-note-variables-01.svg){alt='Value of 65.0 with weight\_kg label stuck on it'}
+
+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](../learners/reference.md#comment) that is ignored by Python.
+Comments allow programmers to leave explanatory notes for other
+programmers or their future selves.
+
+![](fig/python-sticky-note-variables-02.svg){alt='Value of 65.0 with weight\_kg label stuck on it, and value of 143.0 with weight\_lb label stuck on it'}
+
+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
+```
+
+![](fig/python-sticky-note-variables-03.svg){alt='Value of 100.0 with label weight\_kg stuck on it, and value of 143.0 with label weight\_lbstuck on it'}
+
+Since `weight_lb` doesn't "remember" where its value comes from,
+it is not updated when we change `weight_kg`.
+
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+:::::::::::::::::::::::::::::::::::::::  challenge
+
+## Check Your Understanding
+
+What values do the variables `mass` and `age` have after each of the following statements?
+Test your answer by executing the lines.
+
+```python
+mass = 47.5
+age = 122
+mass = mass * 2.0
+age = age - 20
+```
+
+:::::::::::::::  solution
+
+## Solution
+
+```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
+```
+
+:::::::::::::::::::::::::
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+:::::::::::::::::::::::::::::::::::::::  challenge
+
+## Sorting Out References
+
+Python allows you to assign multiple values to multiple variables in one line by separating
+the variables and values with commas. What does the following program print out?
+
+```python
+first, second = 'Grace', 'Hopper'
+third, fourth = second, first
+print(third, fourth)
+```
+
+:::::::::::::::  solution
+
+## Solution
+
+```output
+Hopper Grace
+```
+
+:::::::::::::::::::::::::
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+:::::::::::::::::::::::::::::::::::::::  challenge
+
+## Seeing Data Types
+
+What are the data types of the following variables?
+
+```python
+planet = 'Earth'
+apples = 5
+distance = 10.5
+```
+
+:::::::::::::::  solution
+
+## Solution
+
+```python
+print(type(planet))
+print(type(apples))
+print(type(distance))
+```
+
+```output
+<class 'str'>
+<class 'int'>
+<class 'float'>
+```
+
+:::::::::::::::::::::::::
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+
+
+:::::::::::::::::::::::::::::::::::::::: keypoints
+
+- 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 of `something`.
+- Use `# some kind of explanation` to add comments to programs.
+- Built-in functions are always available to use.
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+