Running Python Code
Learning Goals
- Run Python code from a file.
- Log output to the terminal.
- Run Python code from the Python shell.
- Create a
pipenv
virtual environment. - Run
pytest
tests.
Key Vocab
- Interpreter: a program that executes other programs. Python programs require the Python interpreter to be installed on your computer so that they can be run.
- Python Shell: an interactive interpreter that can be accessed from the command line.
- Data Type: a specific kind of data. The Python interpreter uses these types to determine which actions can be performed on different data items.
- Exception: a type of error that can be predicted and handled without causing a program to crash.
- Code Block: a collection of code that is interpreted together. Python groups code blocks by indentation level.
- Function: a named code block that performs a sequence of actions when it is called.
- Scope: the area in your program where a specific variable can be called.
Introduction
In this lesson, you'll get some practice running Python code, and see a few different ways to check what your code is doing. Make sure to code along to get comfortable in this new environment!
Creating a Python Application
Let's dive right in. To get started on any new Python application, the first
thing we need is a file. Create a new file in the lib/
directory called
app.py
. In this file, add the following:
# lib/app.py
print("Hello world!")
print()
is a built-in Python function that will output a string of text to the
terminal. It's the Python equivalent of console.log()
in JavaScript. It will
print the string "Hello world!" along with a line break at the end.
The line above print()
is a Python comment. In Python, any line that starts with
a #
won't be executed by the interpreter. This is the Python equivalent of
//
in JavaScript.
Running Python Applications
Unlike JavaScript, you won't be running Python applications in the browser. Instead, you'll need to use the Python interpreter to run your code from the terminal. You can check which version of Python you're using with this command:
$ python --version
Python 3.8.13
To run the application, enter the command python filename.py
, where
filename.py
is the relative path to the file you'd like to run:
$ python lib/app.py
Hello world!
Congrats on running your first Python application! π
Displaying Data with "print()"
Now that we have a place to write some code, let's explore a few of Python's built-in options for displaying data in different ways.
Standard print() Statements
By default, print()
includes a newline character at the end of your string.
Try entering several print()
statements in a row in lib/app.py
:
# lib/app.py
print("Hello world!")
print("Hello sun!")
print("Hello sky!")
What do you see when you execute lib/app.py
from the command line?
$ python lib/app.py
# => Hello world!
# => Hello sun!
# => Hello sky!
Choosing your own print() Ending
Let's say you're writing a full paragraph and don't need a newline character
after every sentence. print()
can accommodate that through its optional end
parameter. Try modifying your print()
statements as follows:
# lib/app.py
print("Hello world!", end=" ")
print("Hello sun!", end="!! ")
print("Hello sky!", end="!!!\n")
What do you see when you execute lib/app.py
from the command line with these new
end
strings?
$ python lib/app.py
# => Hello world! Hello sun!!! Hello sky!!!!
end
can be a string of any length, including characters like the newline \n
.
Exploring Python with the Python Interpreter
Python comes with a command line interpreter (often called the "Python shell") for running a Python REPL (read-evaluate-print-loop) in the terminal, which provides similar functionality to the browser console that you're familiar with from JavaScript. Using the Python shell is a great way to quickly test out some code, or check your syntax, without needing to run an entire application.
To use the Python shell, go into the terminal and enter python
:
$ python
Python 3.8.13 (default, Jun 2 2022, 15:59:12)
[Clang 13.1.6 (clang-1316.0.21.2.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
This gives you a prompt where you can enter Python code. Try entering in
print("Hello Python shell")
:
$ python
Python 3.8.13 (default, Jun 2 2022, 15:59:12)
[Clang 13.1.6 (clang-1316.0.21.2.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> print("Hello Python shell")
# => Hello Python shell
Try running a few more expressions in the Python Shell:
>>> first_number = 7
>>> print(first_number)
# => 7
In the code above, we've declared a local variable called first_number
and
assigned it a value of 7
. When we tell the Python shell to print(first_number)
,
we see our local variable's value on the next line.
In Python, it's convention to use underscores ( _ ) to separate words in variables. This is referred to as snake case (as opposed to camel case, which is the convention in JavaScript).
You can exit the Python shell by typing exit()
, or pressing ctrl + d
.
pytest
Tests
Running All the lessons in the Python curriculum use the pytest
library for testing your
Python code.
In this lesson, you'll see a testing/
folder with one file, app_test.py
.
app_test.py
is where we've defined tests specifically for this lesson.
pytest
is a Python library (the Python equivalent of an npm package) that
provides a very simple and clean way to write tests.
pytest
Installing Each lesson in the Python curriculum will contain a file called Pipfile
. This
file contains all of the required Python libraries for your work, and restricts
them to the repository that you're working in.
To install pytest
and any other required libraries, simply navigate to a folder
with a Pipfile
and enter pipenv install
:
$ pipenv install
Pipfile.lock not found, creating...
Locking [dev-packages] dependencies...
Building requirements...
Resolving dependencies...
β Success!
Locking [packages] dependencies...
Building requirements...
Resolving dependencies...
β Success!
Updated Pipfile.lock (cb35ed)!
Installing dependencies from Pipfile.lock (cb35ed)...
π ββββββββββββββββββββββββββββββββ 0/0 β 00:00:00
To activate this project's virtualenv, run pipenv shell.
Alternatively, run a command inside the virtualenv with pipenv run.
Now that your pipenv
virtual environment is ready to use, enter pipenv shell
to start working:
$ pipenv shell
Launching subshell in virtual environment...
. /Users/.../venv/bin/activate
$ . /Users/.../.venv/bin/activate
(phase-3-running-python-code) $
Let's take one of these tests as an example to see pytest
's syntax:
def test_app_py_exists():
assert(path.exists("lib/app.py"))
pytest
expects a function with test
in its name within the testing
folder. We will discuss functions later on in this module. For now, all that
you need to know is that a function performs an action when it's called.
Normally, you would have to call the function yourself with
test_app_py_exists()
, but in this case pytest
is calling all test
functions for you.
Delete the file lib/app.py
to start from scratch so we can try getting these
tests to pass.
To run the tests, you will simply run pytest
from inside of your
phase-3-running-python-code
directory.
$ pytest
============================= test session starts ==============================
platform darwin -- Python 3.10.4, pytest-7.1.2, pluggy-1.0.0
rootdir: /Users/benbotsford/Documents/phase-3-running-python-code
collected 3 items
lib/app.py exists in lib directory F [ 33%]
lib/app.py is executable F [ 66%]
lib/app.py prints "Hello World! Pass this test, please." F [100%]
=================================== FAILURES ===================================
...
=========================== short test summary info ============================
FAILED lib/app.py exists in lib directory - AssertionError: assert False
FAILED lib/app.py is executable - FileNotFoundError: [Errno 2] No such file or di...
FAILED lib/app.py prints "Hello World! Pass this test, please." - FileNotFoundErr...
============================== 3 failed in 0.02s ===============================
pytest
provides all kinds of useful information about what went right and wrong
with our code, so make sure to spend your time reading all the output here! It
will tell you:
- Which tests passed/didn't pass
- Why each failing test failed (the difference between the expected output and the actual output)
- The line number of the failing test
You can also use the -x
flag to tell pytest
to stop running after the first
failing test. This technique is helpful for focusing your attention on one
problem at a time:
$ pytest -x
============================= test session starts ==============================
platform darwin -- Python 3.10.4, pytest-7.1.2, pluggy-1.0.0
rootdir: /Users/benbotsford/Documents/phase-3-running-python-code
collected 3 items
lib/app.py exists in lib directory F
=================================== FAILURES ===================================
_________________________ TestAppPy.test_app_py_exists _________________________
self = <app_test.TestAppPy object at 0x103743a90>
def test_app_py_exists(self):
'''
exists in lib directory
'''
> assert(path.exists("lib/app.py"))
E AssertionError: assert False
E + where False = <function exists at 0x10292e0e0>('lib/app.py')
E + where <function exists at 0x10292e0e0> = path.exists
testing/app_test.py:16: AssertionError
=========================== short test summary info ============================
FAILED lib/app.py exists in lib directory - AssertionError: assert False
!!!!!!!!!!!!!!!!!!!!!!!!!! stopping after 1 failures !!!!!!!!!!!!!!!!!!!!!!!!!!!
============================== 1 failed in 0.01s ===============================
Instructions
To finish this lab, use the print()
in the lib/app.py
file
as described by the tests:
- Use
print()
to display the text on one line: "Hello World! Pass this test, please."
Using pytest
will run the tests. After they are passing, sync your progress
using Git. When your tests are all passing and your work is synced, the lab is
complete!
Conclusion
This lesson covered a good amount of material, but now you should be familiar
with running code in a Python pipenv
environment, and using tools like the
Python Shell and pytest
as well as built-in methods like print()
to
understand what happens when your Python code is running. You'll need all these
tools going forward, so make sure to get practice with all of them as you
progress through this phase!