03 The first Python program

In your work directory, create a directory ~/PHY494/03_python and go to this directory:

mkdir ~/PHY494/03_python
cd ~/PHY494/03_python

Hello World

Using you favorite editor (for instance, atom), create a text file named hello.py in the directory ~/PHY494/03_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

It should ask you for your name: type it and hit Enter. You should be greeted nicely with a message on your screen.

• The line starting with # is a comment and Python ignores everything on the line starting with the comment character.
• Blank lines are ignored.
• name is a variable: it stores a value (more later); variable names are arbitrary (as long as you only use letters, numbers, underscore, don't start with a number, and don't use a name that is already used by Python)
• input() and print() are functions: they take values as input and do something with it (more later).

What's a Python program?

• A text file with content that follows the rules of the Python programming language.

  Customarily, we add the suffix .py to the filename, but this is not required.

• The text file is read by the python interpreter (a program) and executed.

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 3.6.8 |Anaconda, Inc.| (default, Dec 29 2018, 19:04:46)
    [GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)] 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". For example, to add two numbers

>>> 41 + 1
42
>>> 1.5 + 5e-1
2.0

Exit with quit() or Ctrl + d.

Help!

In Python you can often get help by saying

>>> help()
>>> help("something")

For example, get help on the addition operator (and much more):

>>> help("+")

Activity: Python as a calculator

Start python and figure out ² how to compute the left hand sides (the right hand sides are there for you to check your results):³

\begin{align} -1 + 2 &= 1 \\% 102 - 201 &= -99 \\% 12345678987654321 \times 9876543210123456789 &= 121932631979881115785550983112635269\\% 3/2 &= 1.5\\% \frac{1}{1 - 0.9^2} &= 5.263157894736843\\% 1 + \frac{2}{1} + \frac{2^2}{1\cdot2} + \frac{2^3}{1\cdot2\cdot3} + \frac{2^4}{1 \cdot 2\cdot3\cdot4} &= 7.0\\% -3^4 &= -81\\% 2 - 5.5\times 10^{-7} &= 1.99999945\\% 1.672621898\times 10^{-27} \times (3\times10^8)^2 &= 1.5053597082 \times 10^{-10}\\% \sqrt{2} &= 1.4142135623730951\\% \sqrt{-1} &= i\\% (1 + 2i) + (-2 + i) &= -1 + 3i\\% \frac{1 + 2i}{2 - i} &= i\\% \end{align}

More operations

• floor division ("integer division") // ⁴
• remainder %

>>> 13 / 3  # division returns a float
4.333333333333333
>>> 13 // 3  # floor division discards the fractional part
4
>>> 13 % 3  # the % operator returns the remainder of the division
1
>>> 4 * 3 + 1  # result * divisor + remainder
13

Potential problems…

Division by zero raises an error (an "exception"):

>>> 1/0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero

Floating point numbers are not real real numbers (more later): calculating \(10 + 10^{-16} = 10.0000000000000001\) in Python

>>> 10 + 1e-16
10.0

does not give the expected answer!

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.

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 3.6.8 |Anaconda, Inc.| (default, Dec 29 2018, 19:04:46)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.3.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]:

where In [1]: is the prompt.

Use ipython instead of python for interactive work because it is much more userfriendly.

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.⁵

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: ⁶

Windows

Run jupyter qtconsole to show graphics inline (this used to be called ipython qtconsole):

jupyter qtconsole

In ipython ⁵

%run motion.py

macOS

Run ipython

ipython

In ipython ⁵

%run motion.py

Troubleshooting:

• If the above does not show a graph, try running ipython --matplotlib=osx.
• Alternatively, jupyter qtconcole will likely also work (with inline graphs).

Linux

Run ipython

ipython --matplotlib=gtk3

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

In ipython ⁵

%run motion.py

Footnotes