Python Refresher

To be successful in this class, students should be able to:

• Write and execute Python code on the class server

• Use variables, lists, and dictionaries in Python

• Write conditionals using a variety of comparison operators

• Write useful while and for loops

• Arrange code into clean, well organized functions

• Read input from and write output to a file

• Import and use standard and non-standard Python libraries

Topics covered in this module include:

• Data types and variables (ints, floats, bools, strings, type(), print())

• Arithmetic operations (+, -, *, /, **, %, //)

• Lists and dictionaries (creating, interpreting, appending)

• Conditionals and control loops (comparison operators, if/elif/else, while, for, break, continue, pass)

• Functions (defining, passing arguments, returning values)

• File handling (open, with, read(), readline(), strip(), write())

• Importing libraries (import, random, names, pip)

Log in to the Class Server

To log in to isp02.tacc.utexas.edu, follow the instructions for your operating system or ssh client below.

Note

Replace username with your TACC username.

Mac / Linux

Open the application 'Terminal'
ssh username@isp02.tacc.utexas.edu
(enter password)
(enter token)

Windows (use WSL or an SCP client like PuTTY)

Open the application 'PuTTY'
enter Host Name: isp02.tacc.utexas.edu
(click 'Open')
(enter username)
(enter password)
(enter token)

If you can’t access the class server yet, a local or web-based Python 3 environment will work for this guide. However, you will need to access the class server for future lectures.

Try this Python 3 environment in a browser.

Note

For the first few sections below, we will be using the Python interpreter in interactive mode to try out different things. Later on when we get to more complex code, we will be saving the code in files (scripts) and invoking the interpreter non-interactively.

Data Types and Variables

Start up the interactive Python interpreter:

[isp02]$ python3
Python 3.6.8 (default, Aug  7 2019, 17:28:10)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-39)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

Tip

To exit the interpreter, type quit().

The most common data types in Python are similar to other programming languages. For this class, we probably only need to worry about integers, floats, booleans, and strings.

Assign some values to variables by doing the following:

>>> my_int = 5
>>> my_float = 5.0
>>> my_bool = True      # or False, notice capital letters
>>> my_string = 'Hello, world!'

In Python, you don’t have to declare type. Python figures out the type automatically. Check using the type() function:

>>> type(my_int)
<class 'int'>
>>> type(my_float)
<class 'float'>
>>> type(my_bool)
<class 'bool'>
>>> type(my_string)
<class 'str'>

Print the values of each variable using the print() function:

>>> print(my_int)
5
>>> print('my_int')
my_int

(Try printing the others as well). And, notice what happens when we print with and without single quotes? What is the difference between my_int and 'my_int'?

You can convert between types using a few different functions. For example, when you read in data from a file, numbers are often read as strings. Thus, you may want to convert the string to integer or float as appropriate:

>>> str(my_int)      # convert int to string
>>> str(my_float)    # convert float to string
>>> int(my_string)   # convert string to int
>>> float(my_string) # convert string to float
>>>
>>> value = 5
>>> print(value)
5
>>> type(value)
<class 'int'>
>>> new_value = str(value)
>>> print(new_value)
'5'
>>> type(new_value)
<class 'str'>

Arithmetic Operations

Next, we will look at some basic arithmetic. You are probably familiar with the standard operations from other languages:

Operator   Function          Example   Result
+          Addition          1+1       2
-          Subtraction       9-5       4
*          Multiplication    2*2       4
/          Division          8/4       2
**         Exponentiation    3**2      9
%          Modulus           5%2       1
//         Floor division    5//2      2

Try a few things to see how they work:

>>> print(2+2)
>>> print(355/113)
>>> print(10%9)
>>> print(3+5*2)
>>> print('hello' + 'world')
>>> print('some' + 1)
>>> print('number' * 5)

Also, carefully consider how arithmetic options may affect type:

>>> number1 = 5.0/2
>>> type(number1)
<class 'float'>
>>> print(number1)
2.5
>>> number2 = 5/2
>>> type(number2)
<class 'float'>
>>> print(number2)
2.5
>>> print(int(number2))
2

Lists and Dictionaries

Lists are a data structure in Python that can contain multiple elements. They are ordered, they can contain duplicate values, and they can be modified. Declare a list with square brackets as follows:

>>> my_shape_list = ['circle', 'heart', 'triangle', 'square']
>>> type(my_shape_list)
<class 'list'>
>>> print(my_shape_list)
['circle', 'heart', 'triangle', 'square']

Access individual list elements:

>>> print(my_shape_list[0])
circle
>>> type(my_shape_list[0])
<class 'str'>
>>> print(my_shape_list[2])
triangle

Create an empty list and add things to it:

>>> my_number_list = []
>>> my_number_list.append(5)     # 'append()' is a method of the list class
>>> my_number_list.append(6)
>>> my_number_list.append(2)
>>> my_number_list.append(2**2)
>>> print(my_number_list)
[5, 6, 2, 4]
>>> type(my_number_list)
<class 'list'>
>>> type(my_number_list[1])
<class 'int'>

Lists are not restricted to containing one data type. Combine the lists together to demonstrate:

>>> my_big_list = my_shape_list + my_number_list
>>> print(my_big_list)
['circle', 'heart', 'triangle', 'square', 5, 6, 2, 4]

Another way to access the contents of lists is by slicing. Slicing supports a start index, stop index, and step taking the form: mylist[start:stop:step]. Only the first colon is required. If you omit the start, stop, or :step, it is assumed you mean the beginning, end, and a step of 1, respectively. Here are some examples of slicing:

>>> mylist = ['thing1', 'thing2', 'thing3', 'thing4', 'thing5']
>>> print(mylist[0:2])     # returns the first two things
['thing1', 'thing2']
>>> print(mylist[:2])      # if you omit the start index, it assumes the beginning
['thing1', 'thing2']
>>> print(mylist[-2:])     # returns the last two things (omit the stop index and it assumes the end)
['thing4', 'thing5']
>>> print(mylist[:])       # returns the entire list
['thing1', 'thing2', 'thing3', 'thing4', 'thing5']
>>> print(mylist[::2])     # return every other thing (step = 2)
['thing1', 'thing3', 'thing5']

Note

If you slice from a list, it returns an object of type list. If you access a list element by its index, it returns an object of whatever type that element is. The choice of whether to slice from a list, or iterate over a list by index, will depend on what you want to do with the data.

Dictionaries are another data structure in Python that contain key:value pairs. They are always unordered, they cannot contain duplicate keys, and they can be modified. Create a new dictionary using curly brackets:

>>> my_shape_dict = {
...   'most_favorite': 'square',
...   'least_favorite': 'circle',
...   'pointiest': 'triangle',
...   'roundest': 'circle'
... }
>>> type(my_shape_dict)
<class 'dict'>
>>> print(my_shape_dict)
{'most_favorite': 'square', 'least_favorite': 'circle', 'pointiest': 'triangle', 'roundest': 'circle'}
>>> print(my_shape_dict['most_favorite'])
square

As your preferences change over time, so to can values stored in dictionaries:

>>> my_shape_dict['most_favorite'] = 'rectangle'
>>> print(my_shape_dict['most_favorite'])
rectangle

Add new key:value pairs to the dictionary as follows:

>>> my_shape_dict['funniest'] = 'squircle'
>>> print(my_shape_dict['funniest'])
squircle

Many other methods exist to access, manipulate, interpolate, copy, etc., lists and dictionaries. We will learn more about them out as we encounter them later in this course.

Conditionals and Control Loops

Python comparison operators allow you to add conditions into your code in the form of if / elif / else statements. Valid comparison operators include:

Operator   Comparison                 Example   Result
==         Equal                      1==2       False
!=         Not equal                  1!=2       True
>          Greater than               1>2        False
<          Less than                  1<2        True
>=         Greater than or equal to   1>=2       False
<=         Less Than or equal to      1<=2       True

A valid conditional statement might look like:

>>> num1 = 10
>>> num2 = 20
>>>
>>> if (num1 > num2):                  # notice the colon
...     print('num1 is larger')        # notice the indent
... elif (num2 > num1):
...     print('num2 is larger')
... else:
...     print('num1 and num2 are equal')

In addition, conditional statements can be combined with logical operators. Valid logical operators include:

Operator   Description                           Example
and        Returns True if both are True         a < b and c < d
or         Returns True if at least one is True  a < b or c < d
not        Negate the result                     not( a < b )

For example, consider the following code:

>>> num1 = 10
>>> num2 = 20
>>>
>>> if (num1 < 100 and num2 < 100):
...     print('both are less than 100')
... else:
...     print('at least one of them is not less than 100')

While loops also execute according to conditionals. They will continue to execute as long as a condition is True. For example:

>>> i = 0
>>>
>>> while (i < 10):
...     print( f'i = {i}' )       # literal string interpolation
...     i = i + 1

The break statement can also be used to escape loops:

>>> i = 0
>>>
>>> while (i < 10):
...     print( f'i = {i}' )
...     i = i + 1
...     if (i==5):
...         break
...     else:
...         continue

For loops in Python are useful when you need to execute the same set of instructions over and over again. They are especially great for iterating over lists:

>>> my_shape_list = ['circle', 'heart', 'triangle', 'square']
>>>
>>> for shape in my_shape_list:
...     print(shape)
>>>
>>> for shape in my_shape_list:
...     if (shape == 'circle'):
...         pass                    # do nothing
...     else:
...         print(shape)

You can also use the range() function to iterate over a range of numbers:

>>> for x in range(10):
...     print(x)
>>>
>>> for x in range(10, 100, 5):
...     print(x)
>>>
>>> for a in range(3):
...     for b in range(3):
...         for c in range(3):
...             print( f'{a} + {b} + {c} = {a+b+c}' )

Note

The code is getting a little bit more complicated now. It will be better to stop running in the interpreter’s interactive mode, and start writing our code in Python scripts.

Functions

Functions are blocks of codes that are run only when we call them. We can pass data into functions, and have functions return data to us. Functions are absolutely essential to keeping code clean and organized.

On the command line, use a text editor to start writing a Python script:

[isp02]$ vim function_test.py

Enter the following text into the script:

def hello_world():
    print('Hello, world!')

hello_world()

After saving and quitting the file, execute the script (Python code is not compiled - just run the raw script with the python3 executable):

[isp02]$ python3 function_test.py
Hello, world!

Note

Future examples from this point on will assume familiarity with using the text editor and executing the script. We will just be showing the contents of the script and console output.

More advanced functions can take parameters and return results:

def add5(value):
    return(value + 5)

final_number = add5(10)
print(final_number)

15

Pass multiple parameters to a function:

def add5_after_multiplying(value1, value2):
    return( (value1 * value2) + 5)

final_number = add5_after_multiplying(10, 2)
print(final_number)

25

It is a good idea to put your list operations into a function in case you plan to iterate over multiple lists:

def print_ts(mylist):
    for x in mylist:
        if (x[0] == 't'):      # a string (x) can be interpreted as a list of chars!
            print(x)

list1 = ['circle', 'heart', 'triangle', 'square']
list2 = ['one', 'two', 'three', 'four']

print_ts(list1)
print_ts(list2)

triangle
two
three

There are many more ways to call functions, including handing an arbitrary number of arguments, passing keyword / unordered arguments, assigning default values to arguments, and more.

File Handling

The open() function does all of the file handling in Python. It takes two arguments - the filename and the mode. The possible modes are read (r), write (w), append (a), or create (x).

For example, to read a file do the following:

with open('/usr/share/dict/words', 'r') as f:
    for x in range(5):
        print(f.readline())

1080

10-point

10th

11-point

12-point

Tip

By opening the file with the with statement above, you get built in exception handling, and it automatically will close the file handle for you. It is generally recommended as the best practice for file handling.

You may have noticed in the above that there seems to be an extra space between each word. What is actually happening is that the file being read has newline characters on the end of each line (\n). When read into the Python script, the original new line is being printed, followed by another newline added by the print() function. Stripping the newline character from the original string is the easiest way to solve this problem:

with open('/usr/share/dict/words', 'r') as f:
    for x in range(5):
        print(f.readline().strip('\n'))

1080
10-point
10th
11-point
12-point

Read the whole file and store it as a list:

words = []

with open('/usr/share/dict/words', 'r') as f:
    words = f.read().splitlines()                # careful of memory usage

for x in range(5):
    print(words[x])

1080
10-point
10th
11-point
12-point

Write output to a new file on the file system; make sure you are attempting to write somwhere where you have permissions to write:

my_shapes = ['circle', 'heart', 'triangle', 'square']

with open('my_shapes.txt', 'w') as f:
    for shape in my_shapes:
        f.write(shape)

(in my_shapes.txt)
circlehearttrianglesquare

You may notice the output file is lacking in newlines this time. Try adding newline characters to your output:

my_shapes = ['circle', 'heart', 'triangle', 'square']

with open('my_shapes.txt', 'w') as f:
    for shape in my_shapes:
        f.write( f'{shape}\n' )

(in my_shapes.txt)
circle
heart
triangle
square

Now notice that the original line in the output file is gone - it has been overwritten. Be careful if you are using write (w) vs. append (a).

Importing Libraries

The Python built-in functions, some of which we have seen above, are useful but limited. Part of what makes Python so powerful is the huge number and variety of libraries that can be imported. For example, if you want to work with random numbers, you have to import the ‘random’ library into your code, which has a method for generating random numbers called ‘random’.

import random

for i in range(5):
    print(random.random())

0.47115888799541383
0.5202615354150987
0.8892412583071456
0.7467080997595558
0.025668541754695906

More information about using the random library can be found in the Python docs

Some libraries that you might want to use are not included in the official Python distribution - called the Python Standard Library. Libraries written by the user community can often be found on PyPI.org and downloaded to your local environment using a tool called pip3.

For example, if you wanted to download the names library and use it in your Python code, you would do the following:

[isp02]$ pip3 install --user names
Collecting names
  Downloading https://files.pythonhosted.org/packages/44/4e/f9cb7ef2df0250f4ba3334fbdabaa94f9c88097089763d8e85ada8092f84/names-0.3.0.tar.gz (789kB)
    100% |████████████████████████████████| 798kB 1.1MB/s
Installing collected packages: names
  Running setup.py install for names ... done
Successfully installed names-0.3.0

Notice the library is installed above with the --user flag. The class server is a shared system and non-privileged users can not download or install packages in root locations. The --user flag instructs pip3 to install the library in your own home directory.

import names

for i in range(5):
    print(names.get_full_name())

Johnny Campbell
Lawrence Webb
Johnathan Holmes
Mary Wang
Jonathan Henry

Exercises

Test your understanding of the materials above by attempting the following exercises.

• Create a list of ~10 different integers. Write a function (using modulus and conditionals) to determine if each integer is even or odd. Print to screen each digit followed by the word ‘even’ or ‘odd’ as appropriate.

• Using nested for loops and if statements, write a program that iterates over every integer from 3 to 100 (inclusive) and prints out the number only if it is a prime number.

• Create three lists containing 10 integers each. The first list should contain all the integers sequentially from 1 to 10 (inclusive). The second list should contain the squares of each element in the first list. The third list should contain the cubes of each element in the first list. Print all three lists side-by-side in three columns. E.g. the first row should contain 1, 1, 1 and the second row should contain 2, 4, 8.

• Write a script to read in /usr/share/dict/words and print just the last 10 lines of the file. Write another script to only print words beginning with the letters “pyt”.

Additional Resources

• The Python Standard Library

• PEP 8 Python Style Guide

• Python3 environment in a browser

• Jupyter Notebooks in a browser