Difference between revisions of "Python-3.4.3/C4/Writing-Python-Scripts/English"

Objectives

• What is importing
• Write your own Python modules and
• Understand the double underscore name double underscore double equal to within double quotes double underscore main double underscore idiom

System Specifications

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

Prerequisite slide

• Run basic Python commands on the IPython console and
• Use Python modules.

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

Python Modules

We can then make use of these functions by importing modules to other scripts.

def gcd(a, b):

while b:

a, b = b, a%b

return a

if gcd(40, 12) == 4:

print ("Everything is OK")

else:

print ("The GCD function is wrong")

while b:

a, b = b, a%b

return a

Highlight if gcd(40, 12) == 4:

print ("Everything is OK")

else:

print ("The GCD function is wrong")

Please take care of the indentation.

We have included a test case in the script.

This test case will check the gcd function every time the script is run.

ipython3

Type,

ipython3 and press Enter.

%run gcd_script.py

Highlight the output

“Everything is OK”

Type,

percentage run gcd underscore script.py

We get an output which says “Everything is OK”.

What if we want to use the gcd function in some other scripts?

This is possible since every Python file can be used as a module.

import sys

Type,

import sys

Highlight the empty string

This indicates that Python searches for a module when it encounters an import statement.

The standard modules are built into Python itself.

Otherwise it is found in system locations like slash usr slash lib slash python3.4 slash

The first item in the list is an empty string.

It means the current working directory is also searched.

import gcd_script

Highlight Everything is OK

Type,

print("gcd of 187 and 391 is", gcd_script.gcd(187, 391))

Highlight gcd of 187 and 391 is 17

Type,

import gcd underscore script

Since gcd underscore script.py is in the current working directory, import will work directly.

Type the print statement as shown here.

We get the gcd of 187 and 391 as output, which is 17.

This is also executed when we imported gcd underscore script.

The test code is added to check the gcd function.

__name__ variable.

Highlight __name__

To execute the test code when the module is imported to other scripts, we can use double underscore name double underscore variable.

Hereafter I will call this as name variable.

Add if __name__ == "__main__":

after the line return a

Now we will add this variable in the script gcd underscore script.py

Type the following after return a statement

if double underscore name double underscore double equal to inside double quotes double underscore main double underscore colon

Indent the code properly.

%run gcd_script.py

Highlight

Everything is OK

Type,

percentage run gcd underscore script.py

We can see that the test is executed successfully and we get output as Everything is OK

Type,

exit

Type,import gcd_script

We didn’t see any changes.

Note that, once a module is imported, it cannot be imported again in an existing IPython console.

So we will exit the existing IPython console by typing exit.

import gcd_script

Type,

import gcd underscore script

We can see that now the test code is not executed.

Everything is OK is not displayed as output.

block in the file gcd_script.py

It is equal to main only when the file is run as a script.

Hence, all the code under the block, if name double equal to within double quotes main is executed only when the file is run as a Python script.

from gcd_script import gcd

def check_relative_prime(a,b):

if gcd(a,b)==1:

print("Yes, %d and %d are relatively prime" %(a,b))

else:

print("No, %d and %d are not relatively prime" %(a,b))

check_relative_prime(11,3)

Highlight Yes, 11 and 3 are relatively prime

We have imported gcd function from gcd underscore script.

And use it in our calculations without the test code being executed.

Two numbers are relatively prime when their gcd is equal to one.

So we get output as Yes, 11 and 3 are relatively prime.

• As an independent stand-alone script or
• As a Python module which can be imported by other Python scripts or modules.

Summary slide

In this tutorial, we have learnt to,

• Import a module
• Use a script as a module
• Write test condition using the name variable
• Restart IPython3 if a changed module is to be imported again.

Evaluation

1. Which of the following variables contains the locations to search for Python modules
2. A module should contain only one function. True or False

Solutions

1. sys.path contains the locations to search for Python modules.
2. False. A module can contain a wide range of functions.

Acknowledgement

For more details, visit this website.