Lesson-2.4

Lesson-2.4Librarycalendartimethis

Library

A library is a collection of functions that share a common theme. This is a loose definition and will become clear when we start working with a library.

`calendar`

Consider the following problem:

$3000$ $15^{\text{th}}$ August will fall on which day of the week?

Python to the rescue:


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import calendar
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calendar.prmonth(3000, 8)

When the above code is executed, the output is:


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    August 3000
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Mo Tu We Th Fr Sa Su
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             1  2  3
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 4  5  6  7  8  9 10
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11 12 13 14 15 16 17
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18 19 20 21 22 23 24
7
25 26 27 28 29 30 31

15th of August falls on a Friday. Isn't that lovely? It took just two lines of code! calendar is one among several libraries in Python's standard library. A comprehensive list can be found here. Going back to the code, calendar is the name of the library and import is the keyword used to include this library as a part of the code.

calendar is a collection of functions that are related to calendars. prmonth is one such function. It accepts <year> and <month>, as input and displays the calendar for <month> in the year <year>. If we want to use a function in calendar, we must first import the library. Let us see what happens if skip this step:


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# import calendar
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calendar.prmonth(3000, 8)

It gives the following error:


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NameError: name 'calendar' is not defined

To access a function defined inside a library, we use the following syntax:


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<calendar>.<function>(<arguments>)

Another way to solve the problem is to use the function weekday:


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import calendar
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print(calendar.weekday(3000, 8, 15))

The output of the above code is 4. Days are mapped to numbers as follows:

Day	Number
Monday	0
Tuesday	1
Wednesday	2
Thursday	3
Friday	4
Saturday	5
Sunday	6

`time`

Let us now try to answer this hypothetical question:

You are stranded on an island in the middle of the Indian Ocean. The island has a computing device that has just one application installed in it: a Python interpreter. You wish to know the current date and time.

Solution


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from time import ctime
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print('The current time is:', ctime())

The output is:


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The current time is: Fri Apr  2 12:24:43 2021

The syntax of the import statement in line-1 looks different. from is a new keyword. The first line of the code is essentially doing the following: from the library called time import the function called ctime. This way of importing functions is useful when we need just one or two functions from a given library:


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from time import ctime, sleep
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print('Current time is:', ctime())
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print('I am going to sleep for 10 seconds')
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sleep(10)
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print('Current time is:', ctime())

sleep(x) is a function in time that suspends the execution of the program for x seconds. If we would be using several functions in the library, then it is a bad idea to keep importing each of them individually. In such cases, it is good to fall back on importing the entire library.

`this`

As a fun exercise, consider the following code:


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import this

This gives the following output:


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The Zen of Python, by Tim Peters
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3
Beautiful is better than ugly.
4
Explicit is better than implicit.
5
Simple is better than complex.
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Complex is better than complicated.
7
Flat is better than nested.
8
Sparse is better than dense.
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Readability counts.
10
Special cases aren't special enough to break the rules.
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Although practicality beats purity.
12
Errors should never pass silently.
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Unless explicitly silenced.
14
In the face of ambiguity, refuse the temptation to guess.
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There should be one-- and preferably only one --obvious way to do it.
16
Although that way may not be obvious at first unless you're Dutch.
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Now is better than never.
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Although never is often better than *right* now.
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If the implementation is hard to explain, it's a bad idea.
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If the implementation is easy to explain, it may be a good idea.
21
Namespaces are one honking great idea -- let's do more of those!

These are some nuggets of wisdom from Tim Peters, a "major contributor to the Python programming language" [refer]. Some of the points make immediate sense, such as "readability counts".