Understanding python vs python3

Coming from a JavaScript and PHP background, my transition into a Python project was… overwhelming.

Suddenly, I was surrounded by new terms — python, python3, pip, pip3, venv, requirements.txt, pipenv, Pipfile, and Pipfile.lock — none of which I’d ever heard of before.

At first, all of these things completely confused me.

The Mystery of python and python3

While running Python scripts from the terminal, I noticed something strange.
Some developers ran:

python hello.py

python hello.py

While others used:

python3 hello.py

python3 hello.py

It was the first time I’d seen this — and I knew there had to be a reason.
So I started digging, searching the web, and even asking AI tools for clarity.

The Discovery

On older systems like Ubuntu 18.04 or macOS pre-2020, I found this pattern:

$ python --version
Python 2.7.18

$ python3 --version
Python 3.8.10

$ python --version
Python 2.7.18

$ python3 --version
Python 3.8.10

Here’s what that means:

python → runs Python 2
python3 → runs Python 3

Why Do We Have Both?

In the past, many production services were built with Python 2.
When Python 3 came along, it introduced several breaking changes — meaning code written for Python 2 wouldn’t always work in Python 3.

If you simply repurpose the python command to point to Python 3, you might break existing services that rely on older syntax.

Example: When Updating the Alias Breaks Things

Imagine a Linux server from 2014 that has been running smoothly for years.
It runs a nightly cron job to back up some files using a Python script:

# Python 2 script
print "Starting backup..."

data = {'files': 120, 'size': 340.5}
print "Total files:", data['files']
print "Total size:", data['size'], "MB"

# Python 2 script
print "Starting backup..."

data = {'files': 120, 'size': 340.5}
print "Total files:", data['files']
print "Total size:", data['size'], "MB"

The cron job looks like this:

python /usr/local/bin/backup_script.py

python /usr/local/bin/backup_script.py

One day, a system admin decides to modernize the environment and updates the alias:

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 1

sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 1

Now python runs Python 3, not Python 2.

The Next Night…

The backup fails with this error:

File "backup_script.py", line 2
    print "Starting backup..."
          ^^^^^^^^^^^^^^^^^^^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print(...)?

File "backup_script.py", line 2
    print "Starting backup..."
          ^^^^^^^^^^^^^^^^^^^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print(...)?

That’s because in Python 3, print is a function, not a statement.

Quick Syntax Comparison

Version	Print Syntax
Python 2	`print "Hello, world!"`
Python 3	`print("Hello, world!")`

So, while new scripts will work fine, older scripts can break instantly when python suddenly points to a newer interpreter.

The Safe Approach

To avoid conflicts on older systems:

Keep python pointing to Python 2
Use python3 explicitly for new projects

This ensures that both old and new scripts can coexist safely.

Modern Systems

On modern OS versions (like Ubuntu 22.04, macOS 12+, or Windows 10+), this confusion no longer exists:

$ python --version
Python 3.10.12

$ python3 --version
Python 3.10.12

$ python --version
Python 3.10.12

$ python3 --version
Python 3.10.12

Now, both commands typically point to Python 3, since Python 2 reached end-of-life on January 1, 2020.

Final Takeaway

Having this awareness can save hours of debugging and prevent production outages that happen simply because of mismatched Python versions.

When in doubt:

Always check your interpreter with python --version
Use python3 explicitly in new projects
Be careful when changing global aliases on shared systems

Understanding this small difference can make a huge impact when working across environments — and keep you from breaking code that’s been running perfectly for years.