Eroman: add lecture 5

2017-03-04T14:16:04Z

add lecture 5

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= Advanced Testing =

See<br />
https://wiki.itcollege.ee/index.php/I719_Fundamentals_of_Python/testing

= Linting =

Python has a style guide called PEP8.

you can check if your code follows PEP8 using the PEP8 linter.

(you may also install this using your OS's package manager)

<pre class="shell">pip3 install pep8
python3 -m pep8 ~/path/to/your_file.py</pre>
This will print out each error as a line in stdout

<code>path/to/your_file.py:LINE:COLUMN PEP8_ERROR EXPLANATION</code>

for example:

<pre class="shell">btc/btc_price_tests.py:8:80: E501 line too long (648 > 79 characters)
btc/btc_price_tests.py:11:1: E302 expected 2 blank lines, found 1</pre>
There is another tool called <code>flake8</code>, which is installed the same way and checks for problems in your code that will cause runtime problems, as well as the pep8 checking.

I recommend you configure your editor to use flake8

Please see http://blog.sideci.com/2015/12/14/style-guide-of-python-and-lint-tool/ for more information on linting

= Datetime =

see https://docs.python.org/3/library/datetime.html

<source lang="python">from datetime import datetime

# convert a timestamp to python native datetime object
timestamp = 1488430906
d = datetime.fromtimestamp(timestamp)</source>
= TASK 1 =

Print the titles of all the frontpage articles on reddit, with the time in normal format (i.e 11:23) next to it.

resource: https://www.reddit.com/.json<br />
example output

<pre>11:23 Sessions spoke twice with Russian ambassador during Trump\u2019s presidential campaign, Justice officials say \/\/ CAN WE HAVE AN INDEPENDENT INVESTIGATION NOW? Call your Representative and get them on the record, Country or party?</pre>
== NOTE ==

reddit has rate limiting and does not give consistent responses!<br />
download the json data as a json file and use the following code

<source lang="python">import json

json_file = open('frontpage.json')
data = json.load(json_file)
json_file.close()</source>
== Solution ==

<source lang="python">import argparse
import json

from datetime import datetime

def get_time_with_title(article):
timestamp = article['data']['created_utc']
d = datetime.fromtimestamp(timestamp)
time = d.strftime('%H:%M')
title = article['data']['title']
result = '{time} {title}'.format(time=time, title=title)
return result

def print_headlines(path):
with open(path) as json_file:
data = json.load(json_file)
articles = data['data']['children']
for article in articles:
line = get_time_with_title(article)
print(line)

def main():
parser = argparse.ArgumentParser()
parser.add_argument('file_name')
args = parser.parse_args()
print_headlines(args.file_name)

if __name__ == '__main__':
main()</source>
= Classes part 2 =

see "6. Classes" section of https://learnxinyminutes.com/docs/python3/

= lamdbas =

Lamdbas are like functions, but are not necessarily defined with a name. You can assign a lamdba expression to a variable, but if you do this, then you should define a function.

<source lang="python"># do this
def plus_one(x):
return x + 1

# do not do this
plus_one_lambda = lamdba x: x + 1

plus_one(1) # 2
plus_one_lambda(1) # 2</source>
Lamdba expressions are used when a short function is needed as an argument for another function.

Function that take lambda expression often work on some iterable and need to be called on each iteration. For example <code>map</code> is a function found in many programming languages. It take an iterable and returns an iterable of equal dimension (i.e. length), but with each element modified in a some way. In python, the first argument of the map function is a function, which is called on each element in the iterable.

<source lang="python">def square(x):
return x**2

# map returns a special map type that must be converted to a list to print
print(list(map(square, range(4))))
print(list(map(lambda x: x**2, range(4))))</source>
prints:

<pre>[0, 1, 4, 9]
[0, 1, 4, 9]</pre>
= list comprehension =

in modern python, the map function is rarely used. The preferred syntax is the list comprehension. it is like a <code>for</code> loop.<br />
the syntax is the following:

<source lang="python">[x + 1 for x in iterable]</source>
The first part, <code>x + 1</code> in this example, can be anything. Whatever this evaluates to, becomes an element in the resulting list. The rest of the comprehension is like a for loop, but the <code>x</code> variable is used in the first part.

All of the following print the same values

<source lang="python"># Non-functional way, generate a new list
squares = []
for x in range(4):
squares.append(x**2)
print(squares)

# map() functional way
squares = map(lambda x: x**2, range(4))
print(list(squares))

# modern way
squares = [x**2 for x in range(4)]
print(squares)</source>

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Eroman: add lecture 5