Writing and running Python from Spyder

Overview

Teaching: 15 min
Exercises: 0 min

Questions

• How can I write Python programs that persist in time?

• How can I use a Python development environment like Spyder?

Objectives

• Running a python script from the command line

• Starting and quitting Spyder

• Writing and executing a simple python script from Spyder.

• Going back and forth between script and Python REPL.

Python scripts

So far we’ve worked in the REPL and we cannot save our programs.

Instead of the interactive mode, Python can read a file that contains Python instructions. This file is commonly referred to as a Python script.

Python scripts are plain text files (see below for a discussion about plain vs rich text formats). To create a plain text file, you need to use a text editor. Depending on whether you’re using GNU/Linux, MacOS or Windows, you’ll need different software for that. Click on the box below that corresponds to your situation.

Writing text files with nano on macOS or GNU/Linux

nano is a bare-bones text editor that’s available on most GNU/Linux distributions and macOS. nano runs inside a terminal emulator. To create a new Python file with nano, start a terminal emulator (“Terminal” app on macOS) and type:

  nano myfile.py

Useful commands are described at the bottom of the terminal window. The symbol ^ means the Control key. So ^X means hold the Control key and press the x key.

Writing text files with notepad on Windows

Here’s how to use notepad on windows

Text vs. Whatever

We usually call programs like Microsoft Word or LibreOffice Writer “text editors”, but we need to be a bit more careful when it comes to programming. By default, Microsoft Word uses .docx files to store not only text, but also formatting information about fonts, headings, and so on. This extra information isn’t stored as characters and doesn’t mean anything to tools like head: they expect input files to contain nothing but the letters, digits, and punctuation on a standard computer keyboard. When editing programs, therefore, you must either use a plain text editor, or be careful to save files as plain text.

Let’s try to write a simple Python script. Open a new plain text file (with the method described above depending on your operating system), name it for instance myfirstscript.py. Write the following python code and save the file.

print("hello world")
varint = 1
print("Variable 'varint' is a", type(varint))
varstr = "astring"
print("Variable 'varfl' is a", type(varfl))

Now let’s execute this script. In the terminal (macOS/Linux) or the Anaconda prompt (Windows), type

$ python myfirstscript.py
hello world
Variable 'varint' is a <class 'int'>
Variable 'varstr' is a <class 'str'>

Can you guess what happened? Python read the file, executing each line one after the other. This is equivalent to typing the 5 lines in the REPL, except you wrote them once and for all in the file.

Programming in Python in practice

When programming in Python, you will find yourself working inside a text editor most of the time, building your program. You can also go back and forth between the text editor and the REPL to try things out in an interactive way, for instance if you’re unsure about syntax. When you’re happy with your program, you can run it with the command python <yourfile>.py.

This means your need three pieces of software running concurrently:

• A text editor.
• The Python REPL.
• A command-prompt or terminal emulator.

You could go a long way with this, but this quickly goes unwieldly, especially as your programms grow. We now learn about Spyder, which is a software that brings everything under one roof

Using Spyder

Spyder is software that provides a convenient environment to develop Python programs.

You can start it from the Anaconda navigator or from the command line by typing spyder.

On the left is a text editor, on the bottom right a Python REPL. The top right corner is dedicated to displaying documentation.

Spyder is an Integrated Development Environment. It combines a text editor and a python REPL. You can also run Python scripts directly from Spyder by clicking on the green arrow in the taskbar.

In addition, Spyder reports syntax errors in real time, reminds you of the parameters for a given function, includes a debugger, allows to send a selection to the REPL for execution… and more. It’s all about developping in Python in an efficient way.

Creating a new file

• Setting the current dir
• Creating new file

Running the script

Using the REPL

From script to REPL

• Running a line or selection
• Cells

A word on IPython

IPython is a Python REPL that builds on top of the default one to provide more functionalities.

Key Points

• Python scripts are plain text files.

• Spyder is a software that integrates both a text editor and the Python runtime.

• Spyder provides convenience features such as autocompletion, documentation lookup and debugging.

• Spyder is one of many options.