Difference between revisions of "Python-3.4.3/C4/Using-Python-Modules/English"

containing title

Learning Objectives

• Execute Python scripts from command line
• Use import in scripts and
• Import numpy and matplotlib.pyplot modules

System Specifications

• Ubuntu Linux 16.04 operating system
• Python 3.4.3
• IPython 5.1.0 and
• Gedit text editor

Prerequisite slide

• Use plot interactively
• Embellish and save a plot

If not, see the relevant Python tutorials on this website.

What is a module?

A module is a file containing Python definitions and statements.

Modules are used to break down large programs into small manageable and organized files.

Definitions from a module can be imported into other modules or to the main module.

Type, print ("Hello World")

Open any text editor and type,

print inside brackets inside double quotes Hello World

We have created a simple Python script to print Hello World.

Then navigate to the directory where hello.py is saved.

python3 hello.py

It executes the script and gives the output as Hello World.

Let me open the file four underscore plot.py in the text editor.

This file is available in the Code files link of this tutorial.

You can download and use it.

Four plot Problem

python3 four_plot.py

Type,

python3 four underscore plot.py

It gives an error linspace() is not defined.

It means that,

function linspace() is not available in the current namespace.

A namespace is a system for making all the names in a program to be unique.

from numpy import *

Highlight linspace

from numpy import asterisk

When we use asterisk in imports, all the functions and constants are imported from numpy module.

linspace is a function available in numpy.

python3 four_plot.py

Now it gave another NameError: name ‘plot’ is not defined.

Add the following as the second line in our script.

from matplotlib.pyplot import *

We get the output now.

We imported all the required modules using the keyword import.

Import functions

from numpy import linspace, pi, sin

instead of,

from numpy import *

Import functions

instead of,

from matplotlib.pyplot import *

Import Functions

• It is always good to use function names instead of asterisk.
• If we use asterisk to import from a particular module, all the functions will be imported.
• It replaces some existing functions with the same name in the namespace.

delete the first two lines and add the following

from numpy import linspace, pi, sin

from matplotlib.pyplot import plot, legend, annotate

from matplotlib.pyplot import xlim, ylim, title, show

Here, we got the plots for x, -x, sin of x and xsin of x in a single figure.

In this method we imported all the required functions to the current namespace.

Let us see that.

Highlight

numpy.pi in line 3 and line 10, 11

plt.plot in line 4

plt.legend in line 8

plt.annotate in line 9

plt.xlim, plt.ylim in line 10 and 11

plt.show() in line 12

This file is also available in the Code files link of this tutorial.

Notice that we are going to use the name plt instead of matplotlib.pyplot.

plt is used as alias to the module matplotlib.pyplot

Now, we use numpy.pi instead of just pi as we did in four underscore plot.py.

Plot functions are called as

plt.plot()

plt.legend()

plt.annotate()

plt.xlim

plt.ylim

plt.show()

The advantage is that function names in imported modules do not get added to the current namespace.

In order to use a function in an imported module, we need to mention module-name.function-name

Exercise 1

Try this exercise and then resume the video.

Write a python script to plot a sine wave from -2pi to 2pi.

Open sine.py and show it

This file is also available in the Code files link.

In the first line we import the required functions.

The functions linspace, sin and constant pi are imported from the module numpy.

We import the functions plot, legend, show, title, xlabel and ylabel from matplotlib.pyplot.

And the rest of the code is to generate the plot.

python3 sine.py

In the terminal, type

python3 sine.py

Now we can see our sine plot.

Close the terminal.

Let us move further in our topic.

ipython3

Open another terminal and start the ipython interpreter by typing

ipython3

%run four_plot.py

percentage run four underscore plot.py

As before we can see the output with 4 plots.

Python standard library

Some of the standard modules are,

• for Math: math, random
• for Internet access: urllib2, smtplib
• for System and Command line arguments: sys

Python standard library

• for Operating system interface: os
• for regular expression: re
• for compression: gzip, zipfile, tarfile

For more information refer to the below link.

Summary

In this tutorial, we have learnt to,

• Run scripts from command line
• Import modules by specifying the module name followed by an asterisk
• Import only the required functions from modules by specifying the function name
• Use Python standard library.

Evaluation

1. Which among the below is the most correct?
2. How the functions xlim() and ylim() can be imported to the current namespace?

Solutions

1. The option from matplotlib.pyplot import plot is the most correct one.

   Because plot is a function of matplotlib.pyplot module.

2. Functions xlim() and ylim() can be imported to the current namespace as, from matplotlib.pyplot import xlim, ylim

For more details, visit this website.

Thanks for watching.