02 The first Python program

In your work directory, create a directory ~/PHY494/02_python and go to this directory:

mkdir ~/PHY494/02_python
cd ~/PHY494/02_python

Hello World

Using you favorite editor, create a text file named hello.py in the directory ~/PHY494/02_python¹ with the following content:

# hello world in Python

name = input("What's your name? --> ")
print("Hello ", name, ", it's great to see you here.")

Execute ("run") your hello.py program with the python program:

python hello.py

Python interpreter

Python is an interpreted language; you can think of python reading each line in an input file and executing it. Start python on its own:

python

You should see something like

 Python 3.5.3 |Anaconda custom (x86_64)| (default, Mar  6 2017, 12:15:08)
 [GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.57)] on darwin
 Type "help", "copyright", "credits" or "license" for more information.
 >>>

The Python prompt is >>>.

Type commands and execute them with Enter.

Like the shell, Python is a REPL, a "Read-eval-print-loop".

Activity: hello, line by line

Type the commands from hello.py into the interpreter, hitting Enter after every line, and execute your program step by step.

Exit with quit() or Ctrl + d

Basic plotting

Create a file motion.py with content

# plot motion with constant acceleration

import matplotlib.pyplot as plt

a = 1.0    # acceleration
v0 = 0.0   # initial velocity

t, h, n = 0.0, 0.1, 20    # init time, step size, number of steps
ta, xa = [], []           # time and position lists

for i in range(n):
    x = v0*t + a*t*t/2.0
    ta.append(t)
    xa.append(x)
    t = t + h

# plot results
plt.figure(figsize=(4, 4))
plt.plot(ta, xa, '-o', color="red", linewidth=2)
plt.xlabel("t (s)")
plt.ylabel("x (m)")

plt.savefig("motion.png")
plt.show()

You should create a graph similar to the following:

Interactive Python with ipython

The ipython interpreter is like python but with lots of improvements such as TAB-completion, help with command? (one question mark directly following a command) and source code with command?? (two question marks), command line history, and many additional shell-like commands (so-called "magic" commands such as %cd, %ls, %pwd, %run, %time and %timeit — see %magic for help).

Start it with

ipython

It should look like

Python 3.5.3 |Anaconda custom (x86_64)| (default, Mar  6 2017, 12:15:08)
Type 'copyright', 'credits' or 'license' for more information
IPython 6.1.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]:

where In [1]: is the prompt.

Just use ipython instead of python for interactive work.

ipython basics

• To exit, give the exit() command or ^D (Control + D).
• Use arrow keys to move up and down through the history or through multi-line commands.
• Use TAB-completion.
• Use command? or ?command (question mark directly before or after a command or object) to get help
• %run FILE executes FILE similarly to python FILE.²

ipython with matplotlib

In the basic plotting example we used the matplotlib library for plotting and wrote a figure to a file. ipython can show figures interactively but the details depend on your operating system and installed packages. Try the following and see if the figure is displayed on your screen when you run motion.py:

Windows

Run ipython qtconsole to show graphics inline:³

ipython qtconsole

In ipython

%run motion.py

macOS

Run ipython

ipython --matplotlib=osx

In ipython

%run motion.py

Linux

Run ipython

ipython --matplotlib=gtk3

(If gtk3 does not work, try qt5, qt, wx, tk, auto, or just --matplotlib.)

In ipython

%run motion.py

Footnotes

    ipython --matplotlib=qt5
    

(If `qt5` does not work, try `qt`, `wx`, `tk`, `auto`, or just
`--matplotlib`.)

However, this does not always work and might require additional
packages. Please share any insights and solutions!