Python fundamentals#

Python is a very powerful tool for automating tasks that would otherwise be time-consuming or impossible to do by hand or other conventional tools. Here, we’ll go over basic ways to use Python to introduce you to a slice of its potential.

Materials:#

Question 1: Do it yourself!#

Write a message and print it out!

#  INSTRUCTIONS:   Write a message in the quotes.
#  type Shift+Enter to run the cell.
message = ''

print(message)

Python as a calculator#

An incredibly simple yet pivotal role of Python is to perform math calculations (addition, subtraction, multiplication, etc.). We show how to basic action below.

You’ll see the symbol # used often. These are comments, and they are used to write descriptions. Any characters following # are not run or executed.

3 + 4 * 5  # addition and multiplication

23

After we run a cell, an output is displayed below.

You may have noticed from that example that order of operations mattered for that calculation. You can parentheses too if you want to group calculations.

12 / (6 - 4) # division and substraction

6.0

Exponentiation (e.g. \(2^3\)) looks a little bit odd in Python as it uses ** instead of a more traditional ^.

2 ** 3 # exponentiation

8

Question 2: Writing math#

Calculate the following value in Python:

\[\frac{25}{(35 - 3)^3}\]
Solution

Remember to include parentheses when needed, but Python also follows standard order of operations.

25/(35-3)**3

Assigning Variables#

A foundational tool in Python is assigning values to variables. We do this with the = operator.

x = 50 # x is 50

This sets the variable x to be 50, an integer, or int. This value of x is now stored in our notebook, and we can access this value in other cells until the notebook is reset. For instance, subtracting 20 from x prints out a value of 30.

# What if I use x again in a different cell?
x - 20

30

Note

Variables persist between cells once they have been run (executed).

If we ever want to check the value of any variable, we can use the built-in print() command to display the value.

y = 35
print(y)

35

We can also assign the value of one variable to another variable. If we execute x = y, x takes the current value of y and assigns that to x.

Warning

y will be unaffected by this assignment. x = y should be interpretted as “let x take the current value of y”.

x = y
print(x)
print(y)

35
35

If we change y to be a different value, x will be unaffected.

y = 3.8
print(x) # will not always be the same value as y
print(y)

35
3.8

Variables only change value when something is assigned to them.

They are not like spreadsheets where a cell can depend on another and update automatically.

Question 3: Swapping values#

Given the code below, what is the value of the variable swap by the end of the block?

x = 1.0
y = 3.0
swap = x
x = y
y = swap
Solution

The only statements here that effect the value of swap are x = 1.0 and swap = x.

1.0

What’s in a name? Variable name conventions

  • Use only letters, digits, and underscores (_)

  • can use camelcase (capitalizeEveryWord)

  • Start with a letter (typically lower case)

  • Variable names are case sensitive

  • Use meaningful names!

Variables must be created before they are used. Otherwise, Python will throw an error.

print(z) # we haven't initialized z yet!

NameError: name 'z' is not defined

In the following example, we specify a value for a variable after we print it. This will not work. Within a cell, the statements are executed from top to bottom. If the Python interpreter finds an error, execution stops.

print(last_name) # last name does not exist yet
# What happens if I try to correct my error in the same cell?
last_name='Montoya'
print(last_name)

If we want to increase a value of a variable by a certain value, we can use the += operator. Here, we add 2 to the current value of x.

print(x) # before incrementing
x += 2
print(x) # after incrementing

35
37

This is equivalent to running x = x + 2. We can also do similar operations with subtraction, multiplication, and division.

x = 8
x -= 2 # 8 - 2
print("subtraction:", x)
x *= 4 # 6 * 4
print("multiplication:", x)
x /= 6 # 24 / 6
print("division:", x)

subtraction: 6
multiplication: 24
division: 4.0

Question 4: Using variables#

a has been initialized to be 25. Assign variable b to be 5 less than a without using b = 20. Print the value of b.

a = 25
# write your code here:
Solution

We can use variable a to assign value of a-5 to b.

a = 25
b = a - 5
print(b)

Strings#

So far, we have only assigned integer values to variables. We can also assign values characters to variables. These are called strings. You can specify a string by putting text within either single quotation ('single') or double quotation marks ("double").

my_name = "Inigo Montoya"
print(my_name)

Inigo Montoya

We can print out a string directly in print(), as well.

print("My name is")
print(my_name) # will print on a second line

My name is
Inigo Montoya

We can also print out multiple values in a single print() statement.

print("My name is", my_name) # print() adds a space between the values

My name is Inigo Montoya

If we print out a string with an integer variable, it will convert the integer to a string in order to print it.

num_balloons = 25
print("I would like", num_balloons, "balloons.")

I would like 25 balloons.

Question 5: String syntax#

What will happen if you run the following code?

last_name = Montoya
print(last_name)
Solution

We receive a NameError because we have not defined the varable Montoya previously. You may also see additional text describing more details about the error, such as where it occurred.

NameError: name 'Montoya' is not defined

Data Types#

  • integers (int) represent positive or negative whole numbers like 3 or -512

  • floating point numbers (float) represent real numbers like 3.14159 or -2.5

  • character strings (str) are text

    • written with single or double quotes (matching)

    • quotations aren’t printed when the string is displayed

# Find the type with function type()
print(type(52))
print(type("Inigo Montoya"))
print(type(3.14))

# notice we are nesting functions -> type() is inside of print()

<class 'int'>
<class 'int'>
<class 'str'>
<class 'float'>

Combining and adding data types#

The + operator concatenates (adds) strings together. However if you try to add an integer and a string, you will receive an error.

print("several" + " concatenated" + " strings") # need to manually add spaces when concatenating

several concatenated strings

print(1 + "2") # adding string to int doesn't work

TypeError: unsupported operand type(s) for +: 'int' and 'str'

We can convert an int to a string with the str() function to combine them. We must convert numbers to strings or vice versa when operating on them. Consistency is key!

print(1+int('2')) # convert string to int to do addition
print(str(1)+'2') # convert int to string to concatenate

3
12

However, we can mix integers and floats freely in operations. This will result in the final value being a float.

print('half is', 1/2.0
print('three squared is', 3.0 ** 2)

half is 0.5
three squared is 9.0

Length#

Strings have length (how many characters long they are), which can find with len(). Floats and ints do not have length.

print( len("a long string") ) # spaces count in length

13

print( len(3.1415) ) # will get an error

Division Types with numbers

  • // operator performs integer floor division (rounds down to nearest integer)

  • / operator performs floating point division (returns a number with a decimal point)

  • % modulo operator returns the remainder from integer division

print(5//3)
print(5/3)
print(5%3)

1
1.6666666666666667
2

Question 6: Picking between data types#

Choose the type (int, float, str) that each of these descriptions should be:

  1. Time elapsed from the start of the year until now in days.

  2. Serial code of a piece of lab equipment

  3. A lab specimen’s age

Solution

  1. int if only considering full days, float otherwise.

  2. str: Identifiers can often have letters or leading zeros.

  3. Depends on the specimen. If using countable units, int, otherwise float.

Question 7: Quadratic formula#

A quadratic equation has the following form:

\[0 = ax^2 + bx + c\]

We can use the quadratic forumula (below) to find the roots of a quadratic equation.

\[x = \frac{-b\pm\sqrt{b^2-4ac}}{2a}\]

Create variables \(a\), \(b\), and \(c\) with the value of \(4\), \(-25\), and \(20\), respectively.

Calculate the values of \(x\) for a quadratic equation with \(a=4\), \(b=-25\), and \(c = 20\). Remember to calculate the values for both plus and minus (\(\pm\)).

# your code below:
Solution
# initialize my variables
a = 4
b = -25
c = 20

# positive side
x_p = (-b + (b**2-4*a*c)**0.5)/(2*a)

# negative
x_m = (-b - (b**2-4*a*c)**0.5)/(2*a)

print(x_p)
print(x_m)

Question 8: Converting data types#

first  = 1.0
second = "1"
third  = "1.1"

Which of the following will return the floating point number 2.0?

# first + float(second)          # choice a
# float(second) + float(third)   # choice b
# first + int(third)             # choice c
# first + int(float(third))      # choice d
# int(first) + int(float(third)) # choice e
# 2.0 * second                   # choice f

2.0

Reference and Resource

This lesson is adapted from Software Carpentry.