List comprehension
simple list comprehension: powers of 2
use list comprehension to compute a list containing the powers of 2:
i.e. [2**0, 2**1, 2**2, 2**3, ..., 2**15]
NOTE: the code should be one just line
reminder: list comprehensions look like[ _____ for _____ in ____ ]where you fill in the blanks
[2**i for i in range(16)]
list comprehension with filtering
take the sentnence "everything shuold be as simple as possible but no simpler" (this is a qoute by albert einstein).
use list comprehension to
- take only words that are shorter than 6 letters
- convert words to uppercase
expected output:
['BE', 'AS', 'AS', 'BUT', 'NO']
reminder: list comprehension with filtering has the form
[ _____ for ____ in ____ if ____ ]
### useful: the qoute to work on
words = "everything should be as simple as possible but no simpler".split()
[word.upper() for word in words if len(word) < 6]
max length of words in a list
using JUST ONE LINE, find the length of the longest word in a given sentence
hints - use the functions str.split(), max() and len():
- break the sentence into words using the function
str.split() - use list comprehension to compute the length of all words
- use the function
maxto get the maximum length from the list of lengths
### useful: sentence to work on from "life of brian"
brian = """“
Alright, but apart from the sanitation, the medicine, education, wine, public order,
irrigation, roads, the fresh-water system, and public health,
what have the Romans ever done for us?”
"""
max([len(word) for word in brian.split()])
max lengh of words, and the actual word too
build on the previous exercise
now, get the length of the longest word as well as the actual word
expected output for the life of brian qoute:
[11, 'sanitation']
or alternatively:
[[11, 'sanitation,'], [11, 'irrigation,'], [11, 'fresh-water']]
# method one
result = max([ [len(word), word] for word in brian.split()])
print(result)
# method two
longest_word = max([len(word) for word in brian.split()])
result = [[len(word), word] for word in brian.split() if len(word) == longest_word]
print(result)
multiple comprehensions
for the list of “interesting” angles: 0, 30, 60, 90, 120, 150, 180
a) compute the list of radian values using list comprehension
0 --> 0
30 --> 30 *math.pi/180
60 --> 60 *math.pi/180
x --> x *math.pi/180
180 --> 180*math.pi/180
b) compute the math.sin value for each of the rad from (a) using list comprehension
0 --> math.sin(0)
x --> math.sin(x)
c) use the format "{:.2f}" and list comprehension to compute nicely formatted strings for the numbers in (b)
x --> "{:.2f}".format(x)
expected output:
['0.00', '0.50', '0.87', '1.00', '0.87', '0.50', '0.00']
import math
interesting = [0, 30, 60, 90, 120, 150, 180]
rads = [deg * math.pi / 180 for deg in interesting]
sins = [math.sin(rad) for rad in rads]
strs = ["{:.2f}".format(x) for x in sins]
print(strs)
dictionary comprehension
a restaurant stores the menu and prices of items in a dictionary. now it needs to double the prices of all the items. can you do this in one line with a dictionary comprehension?
menu = {
'steak' : 150,
'falafel' : 20,
'pizza' : 59,
'hamburger' : 63,
'chips' : 18,
'salad' : 30
}
reminders:
- dictionary comprehension has the form
{ ____ : ____ for ___ in ____ }- remember the
.items()function of dictionaries
### useful: here is the menu
menu = {
'steak' : 150,
'falafel' : 20,
'pizza' : 59,
'hamburger' : 63,
'chips' : 18,
'salad' : 30
}
{ item:price*2 for item,price in menu.items() }
dictionary comprehension with filtering
doubling all prices turns out to be too expensive !
we want raise prices in a different way:
- remove all the items that already cost 100 NIS or more from the menu
- all other items should have their price increased by 20%
- make sure every price is in whole shekels (no pennies / agorot)
hint:
- instead of removing, just keep the ones that are cheaper than 100
- use the function
round()
expected output:
{'falafel': 24, 'pizza': 71, 'hamburger': 76, 'chips': 22, 'salad': 36}
{ item:round(price*1.2) for item,price in menu.items() if price < 100 }
cost of a meal
People came into our restaurant and ordered a meal for the table:
order = ['steak', 'steak', 'steak', 'falafel', 'chips', 'salad', 'salad']
lets use the original menu
menu = {
'steak' : 150,
'falafel' : 20,
'pizza' : 59,
'hamburger' : 63,
'chips' : 18,
'salad' : 30
}
and using list comprehension calculate the cost of the meal hint:
- use list comprehension
- use the function
sum()
expected output:
548
also see This video
### useful: the order
order = ['steak', 'steak', 'steak', 'falafel', 'chips', 'salad', 'salad']
sum([menu[food] for food in order ])
😵Create a matrix using list comprehension
use list comprehension to create an NxM marix with numbers going from 0 to NxM
hint: you need nested list comprehension - list comprehension within list comprehension
example with N=6 M=4
[[0, 1, 2, 3],
[4, 5, 6, 7],
[8, 9, 10, 11],
[12, 13, 14, 15],
[16, 17, 18, 19],
[20, 21, 22, 23]]
nrows = 6
ncols = 4
[ [i for i in range(i, i+ncols)] for i in range(0, nrows*ncols, ncols)]
set comprehension
use set comprehension to find all the unique words in the song “knights of the round table” by monty python
- sort the words in the set using the
sorted()function- see if there are words that look similar
- remove duplicates by turning all words into lowercase
- use the
.replace('.', '')function to remove the coma'.'characters in words
- alernatively remove special characters using the
str.translate()andstr.maketrans()functions- print all the unique words in this song
### useful: song lyrics
knights_of_round_table = """
We're knights of the round table
We dance whene'er we're able
We do routines and scenes
With footwork impeccable.
We dine well here in Camelot
We eat ham and jam and spam a lot.
We're knights of the Round Table
Our shows are formidable
But many times we're given rhymes
That are quite unsingable
We're opera mad in Camelot
We sing from the diaphragm a lot.
In war we're tough and able.
Quite indefatigable
Between our quests we sequin vests
And impersonate Clark Gable
It's a busy life in Camelot.
I have to push the pram a lot.
"""
sorted(
{word.lower().replace('.', '') for word in knights_of_round_table.split()}
)
using sets to test a password
can you use a set to check that a password has one of the following special characters:
special_chars = """!@#$%^&*()-=_+`~;'"|/.,<>?"""
hint: it requires only one line, no loops and no list comprehension
special_chars = """!@#$%^&*()-=_+`~;'"|/.,<>?"""
passw = 'abcABC123!@#'
set(passw) & set(special_chars)
pprint
sometimes regular old print() makes it difficult to read a data structure.
for instance take a look at the following code
people = [
['adva', 28, 'physicist'],
['ben', 25, 'hairdresser'],
['carmel', 30, 'mountaineer'],
['daria', 32, 'doctor'],
['edward', 45, 'king']
]
print(people)
- whats weird or annoying about the output of the program? try it yourself
- run the following code which imports a pretty print function called
pprintfrom pprint import pprint pprint(people)any differece?
- read the documentation of the pprint function https://docs.python.org/3.7/library/pprint.html
- try using pprint with parameter
depth=1 - try using pprint with parameter
indent=4
people = [
['adva', 28, 'physicist'],
['ben', 25, 'hairdresser'],
['carmel', 30, 'mountaineer'],
['daria', 32, 'doctor'],
['edward', 45, 'king']
]
print(' ', 1, '-' * 20, sep='\n')
print(people)
# 2
print(' ', 2, '-' * 20, sep='\n')
from pprint import pprint
pprint(people)
# 4
print(' ', 4, '-' * 20, sep='\n')
pprint(people, indent=3, depth=1)
# 5
print(' ', 5, '-' * 20, sep='\n')
pprint(people, indent=4)
zip
basic use of the zip function
look at the following code, read it, can you figure out what it does?
fruits = ["apricots", "banannas", "cherries"]
colors = ["pink", "yellow", "red"]
for fruit, color in zip(fruits, colors):
print(fruit, "are", color)
-
copy the code below and try it for yourself to see if your were right
-
read the documentation for the zip function https://docs.python.org/3.7/library/functions.html#zip
- we have data about people in columns
first_name_column = ['adva', 'ben', 'carmel'] age_column = [28, 25, 30] job_column = ['physicist', 'hairdresser', 'mountaineer']we would like to print data about each person with one row per person, in csv format
expected output:
adva, 28, physicist ben, 25, hairdresser carmel, 30, mountaineer daria, 32, doctor edward, 45, king -
now we want the data as a list of lists, that is a list of rows, where each row is the data for a person
expected output ( use
pprint()):[['adva', 28, 'physicist'], ['ben', 25, 'hairdresser'], ['carmel', 30, 'mountaineer'], ['daria', 32, 'doctor'], ['edward', 45, 'king']]hint: we provide a few solutions, its worthwhile to go through them
### useful: data
first_name_column = ['adva', 'ben', 'carmel', 'daria', 'edward']
age_column = [28, 25, 30, 32, 45]
job_column = ['physicist', 'hairdresser', 'mountaineer', 'doctor', 'king']
from pprint import pprint
# 3
for name, age, job in zip(first_name_column, age_column, job_column):
print(name, age, job, sep=', ')
from pprint import pprint
# 4 solution A
data = [ [name, age, job] for name, age, job in zip(first_name_column, age_column, job_column)]
print()
pprint(data)
# 4 solution B
data = [ row for row in zip(first_name_column, age_column, job_column)]
print()
pprint(data)
# 4 solution C
data = list(zip(first_name_column, age_column, job_column))
print()
pprint(data)
unzipping
There’s a question that comes up frequently in forums for new Pythonistas: “If there’s a
zip()function, then why is there nounzip()function that does the opposite?”
The reason why there’s no
unzip()function in Python is because the opposite ofzip()is… well,zip(). So, how do you unzip Python objects?
here’s a trick:
rows = [['adva', 28, 'physicist'],
['ben', 25, 'hairdresser'],
['carmel', 30, 'mountaineer'],
['daria', 32, 'doctor'],
['edward', 45, 'king']]
# notice the * unpacking operator
name_col, age_col, job_col = zip(*rows)
print(name_col)
print(age_col)
print(job_col)
notice how it turns rows of data into columns of data?
-
now, using just one line, calculate a variable called
columnsfromrowsusingzip().print
columns( usepprint()) -
now, using just one line, reverse
columnsback into rows usingzip()
you’ve learned how to transpose between columns and rows of data
### useful: data
rows = [['adva', 28, 'physicist'],
['ben', 25, 'hairdresser'],
['carmel', 30, 'mountaineer'],
['daria', 32, 'doctor'],
['edward', 45, 'king']]
print(' ', 1, '-' * 20, sep='\n')
columns = list(zip(*rows))
pprint(columns)
print(' ', 2, '-' * 20, sep='\n')
rows2 = list(zip(*columns))
pprint(rows)
Generators and coroutins
even_numbers() infinite lazy list
- write a function called
even_numbers()that returns an infinite lazy list with all the even numbers0, 2, 4, 6, 8, ...hint: write a generator function (function with ‘yield’ statement) with an infinite loop
-
call
even_numbers()and use thenext()function to print the first 3 values from the list - use a combination of the
zip()function with therange(20)function to create a lazy list of the next 20 numbers. use aforloop to iterate over the shorter list
# 1
def even_numbers():
y = 0
while True:
yield y
y += 2
# 2
evens = even_numbers()
print(next(evens))
print(next(evens))
print(next(evens))
# 3
for num, _ in zip(evens, range(20)):
print(num)
line_reader()
did you ever think about how
for line open('myfile')actually works?
imagine files had only one function: .readline()
can you write a generator function called line_reader so that you could still write an easy for loop over the file’s content?
- use the
silly_open()function provided below to get file objects that indeed only have a.readline()function and nothing else
after your write your line_reader function you should be able to write code like this
f = silly_open('knights.txt')
for line in line_reader(f):
print(line)
### useful:
### 1) create a 'knights.txt'
### 2) create a silly_open() function that returns a file with just a .readline() function
with open('knights.txt', 'w') as f:
f.write(knights_of_round_table)
def silly_open(filename):
# define a "Stupid File" which only has a 'readline()' function
class StupidFile:
def __init__(self, f):
self.__f = f
def readline(self):
return self.__f.readline()
def close(self):
self.__f.close()
# open the file 'knights.txt' for reading
f = StupidFile(open(filename, 'r'))
return f
### notice that the following for loop can't work
# for line in silly_open('knights.txt'):
# print(line)
def line_reader(f):
while True:
line = f.readline()
if line == '':
break
yield line
### useful: test for your line_reader() function
f = silly_open('knights.txt')
for line in line_reader(f):
print(line.rstrip())
yield triangles
remember the function triangle(n) that prints a triangle?
def triangle(n):
for i in range(1, n+1):
print("*" * i)
1) rewrite this function with yield so that it yields every line of the triangle instead of printing it 2) do a for loop on the function and print each line 3) take all the lines from the functions into a list, print the list
# 1
def yield_triangle(n):
for i in range(1, n+1):
yield "*" * i
# 2
for line in yield_triangle(5):
print(line)
# 3
lines = list(yield_triangle(5))
print('\n'.join(lines))
yield passwords
can you write a generator function (function with ‘yield’ statement) that:
- uses input() to ask the user for a password
- checks the password (not too long, not too short, has both upper and lower characters)
def yield_password(min_len=4, max_len=12):
while True:
passw = input('make a new password: ')
if len(passw) < min_len:
yield [False, 'too short', passw]
elif len(passw) > max_len:
yield [False, 'too long', passw]
elif passw.islower():
yield [False, 'need uppercase letters', passw]
elif passw.isupper():
yield [False, 'need lowercse letters', passw]
elif not set(passw) & set("""!@#$%^&*()_+?><{}[]\|"""):
yield [False, 'need special characters', passw]
else:
yield [True, 'excellent', passw]
break
for ok, message, passw in yield_password():
print(message)
if ok:
break