Data Science Toolbox

Question 1

How do you define a function?

Answer 1

def functionname ():

expression

expression

functionanme() will output expression

Question 2

how you do define a function with a parameter?

Answer 2

def fxnname(parameter):

expression

expression

e.g.

def square(value):

new_value=value**2

print(new_value)

This will square any argument (value) you input in the fxn square()

Question 3

Define a fxn with a parameter, but instead of printing the value, return it

Answer 3

def square(value):

new_value = value ** 2

return new_value

Now can use fxn to assign the result to a new variable, e.g. num = square(4)

Question 4

What are docstrings? 4 facts

Answer 4

• Describe what your function does
• Serve as documentation for your function
• Placed in the immediate line after the function header
• In between triple double quotes “””

e.g.

def square(value):

”"”Return the square of a value”””

new_value = value**2

return new_value

Question 5

When you assign a variable to a function that prints a value but does not return a value, what type of value will the variable be?

e.g. y = print(x)

Answer 5

NoneType

Question 6

How do you define a function with more than one parameter?

Answer 6

def fxnname(parameter1, parameter2)

expressions

e.g.

def raise_to_power(value1, value2)

new_value = value1 ** value2

return new_value

Question 7

What can you use to make functions return multiple values?

Facts about the answer to above

Answer 7

Tuples

e.g. even_nums = (2, 4, 6)

• Similar to list, except can’t modify values (immutable), and constructed using parenthesis
• Can unpack tuples into several variables:

a, b, c = even_nums

print(a) will output 2, print(b) will output 4, print(c) will output 6

• Can access tuple elements just like lists (zero-indexing)

second_num = even_nums[1]

print(second_num) will output 4

Question 8

Use a tuple to return multiple values. Complete the code below

def raise_both(value1, value2)

’'’Raise value1 to the power of value2 and vice versa.”””

new_value1 = value1 ** value2

new_value2 = value2 ** value 1

Answer 8

def raise_both(value1, value2)

’'’Raise value1 to the power of value2 and vice versa.”””

new_value1 = value1 ** value2

new_value2 = value2 ** value 1

new_tuple = (new_value1, new_value2)

return new_tuple

Question 9

Define Scope

Name and define 3 types of scope

Answer 9

Scope is the part of the program where an object or name may be accessible

3 Types:

1. Global Scope: defined in the main body of a script
2. Local scope: defined inside a function. Once execution of fxn is done any name inside the function ceases to exist, so can’t access those names outside the function definition
3. Built-in scope: names in the pre-defined built-ins module, e.g. print()

Question 10

new_val = 10

def square(value):

”"”Returns the square of a number”””

new_value2 = new_val ** 2

return new_value2

new_val = 20

square(3)

What will be the output?

Answer 10

400

The global value accessed is the one at the time the function is called–not the value when the function is defined.

Question 11

How do you alter the value of a global name within a function call?

Answer 11

using global variable

new_val = 10

def square(value):

”"”Returns square of a number.”””

global new_val

new_val = new_val ** 2

return new_val

new_val will now output 100

*If you don’t use global, it would output 10

Question 12

How do you print the names in the module builtins?

Answer 12

import builtins

dir(builtins)

Question 13

def raise_val(n):

”"”Return the inner function.”””

def inner(x):

”"”Raise x to the power of n.”””

raised = x ** n

return raised

return inner

square = raise_val(2)

square(2)

What will be the output?

Answer 13

4

The program created a function “square” that squares any number.

Similarly, can do cube = raise_val(3) to create a function that cubes any number, so cube(4) will return 64

Question 14

How do you change names in an enclosing scope in a nested function?

Answer 14

use nonlocal variable

def outer():

n = 1

def inner():

nonlocal n

n = 2

print (n)

inner()

print(n)

Now, outer() will output 2

Question 15

In which order are scopes searched?

Answer 15

Local scope, Enclosing functions, Global, Built-in

“LEGB rule”

Question 16

Add a default argument of 1 for pow in the following code

def power(number, pow)

”"”Raise number to the power of pow.”””

new_value - number ** pow

return new_value

After modifying the code, what would the following output?

power(9, 2)

power(9, 1)

power(9)

Answer 16

def power(number, pow=1)

”"”Raise number to the power of pow.”””

new_value - number ** pow

return new_value

power(9, 2): 81

power(9, 1): 9

power(9): 9

Question 17

How do you add flexible arguments to a function?

Flexible arguments allows you to pass any number of arguments to the function

Answer 17

Use *args

e.g.

def add_all(*args)

”"”Sum all values in *args together.”””

sum_all = 0

for num in args:

sum_all += num

return sum_all

*args creates a tuple with all the arguments then the for loop iterates over them

Question 18

How do you make a flexible argument for key:value style pairs (keyword arguments)?

Answer 18

use **kwargs (keyword arguments)

def print_all(**kwargs):

”"”Print out key-value pairs in **kwargs.”””

for key, value in kwargs.items():

print(key + “:” + value)

This creates a dictionary kwargs. The output will look like:

print_all(name=’dumbledore’, job=headmaster’)

job: headmaster
name: dumbledore

*Note: can add as many key/value pairs as you like

Question 19

Write a lambda function

Answer 19

functionname = lambda argument1, argument2: expression

e.g.

raise_to_power = lambda x, y: x ** y

Output: raise_to_power(2,3) = 8

Question 20

What is the map function?

Answer 20

map(func, seq)

Applies the function to all elements in the sequence

Question 21

Filter function

Answer 21

filter(function, sequence)

function: function that tests if each elements of a sequence true or false
sequence: sequence which needs to be filtered

Can be used to filter out certain elements from a list

Question 22

Reduce function

Answer 22

from functools import reduce

reduce(function, seq)

It applies a rolling computation to sequential pairs of values in a list.

e.g.

product = reduce((lambda x, y: x*y), [1, 2, 3, 4])

Output: 24

(1*2*3*4)

Question 23

try-except clause

Answer 23

Exceptions are caught during execution

Python will try to run the code following try. If there’s an exception, it will run the code following except

e.g.

def sqrt(x):

”"”Returns the square root of a number.”””

try:

return x ** 0.5

except:

print(‘x must be an int or float’)

*If try to put a string in function sqrt, will get except message

Question 24

raise an error

Answer 24

Used when wouldn’t get an error message from python, but the output is not desired

e.g. for a sqrt function, we don’t want people to input negative numbers

def sqrt(x):

”"”Returns the square root of a number.”””

if x<0:

raise ValueError(‘x must be non-negative’)

try:

return x ** 0..5

except TypeError:

print(‘x must be an int or float’)

Question 25

Define iterable

Define iterator

Answer 25

Iterable: an object with an associated iter() method, e.g. lists, strings, dictionaries.

Applying iter() to an iterable creates an iterator

Iterator: produces next value with next()

Question 26

Create an iterator named “it” from the following iterable:

word = ‘Da’

What is the output of

next(it)

next(it)

next(it)

Answer 26

it = iter(word)

First next(it): ‘D’

Second next(it): ‘a’

Third next(it): Stop iteration error

Question 27

How do you print all the values of an iterator?

Use:

word = ‘Data’

it = iter(word)

Answer 27

Use the * operator

print(*it)

Output: D a t a

*Note if you use print(*it) again, no values will print. Would need to redefine the iterator

Question 28

file = open(‘file.txt’)

it = iter(file)

print(next(it))

What will be the output?

print(next(it))

What will be the output?

Answer 28

Output:

First line of file

Second line of file

Question 29

range function

Answer 29

range(start, stop)

*stop is 1 + the highest number that will be in the range. Range starts from zero (unless a start is specified)

This creates a range object with an iterator that produces the values until it reaches the limit

e.g.

for i in range(5):

print(i)

0
1
2
3
4

Question 30

Make a range that produces values from 10 to 20 and assign it to “values”.

Create a list from “values”.

Sum all the numbers in “values.”

Answer 30

values = range(10, 21)

list(values)

sum(values)

Question 31

How do you change the beginning index number used by the enumerate()?

Answer 31

enumerate(seq, start = number)

Question 32

zip function

Answer 32

zip(seq, seq)

Accepts an arbitrary number of iterables and returns an iterator of tuples (a zip object)

e.g.

list1 = [1, 2, 3, 4]

list2 = [5, 6, 7, 8]

z = zip(list1, list2)

z_list=list(z) (To turn it from zip object to list)

print(z_list)

[(1, 5), (2, 6), (3, 7), (4, 8)]

Question 33

Use a for loop to print the objects in zip(list1, list2)

list1 = [1, 2, 3, 4]

list2 = [5, 6, 7, 8]

Answer 33

for z1, z2 in zip(list1, list2):

print(z1, z2)

Output:

1 5

2 6

3 7

4 8

Question 34

Print all the elements of zip(list1, list2) using the splat (*) operator

list1 = [1, 2, 3, 4]

list2 = [5, 6, 7, 8]

Answer 34

list1 = [1, 2, 3, 4]

list2 = [5, 6, 7, 8]

z = zip(list1, list2)

print(*z)

Output:

(1, 5) (2, 6) (3, 7) (4, 8)

Question 35

Unpack the tuples produced by zip() with * into list_1 and list_2

list1 = (1, 2, 3, 4)

list2 = (5, 6, 7, 8)

z = zip(list1, list2)

Answer 35

list_1, list_2 = zip(*z)

This will create two tuples that are like the original list1 and list2:

list_1 = (1, 2, 3, 4)

list_2 = (5, 6, 7, 8)

Question 36

What can you do when there’s too much data to hold in memory?

Answer 36

Load data chunks

e.g. Summing column ‘x’ in data.csv

import pandas as pd

total = 0

for chunk in pd.read_csv(‘data.csv’, chunksize = 1000)

total += sum(chunk[‘x’])

print(total)

Question 37

list comprehension syntax

Answer 37

newlist = [output expression for iterator variable in iterable if predicate expression]

The output expression determines the values you wish to create. The for loop iterates over a sequence. The if expression is optional. *Note can also add a second for loop to similate nested for loops. *Note2: can also use if statement in output expression.

List comprehension helps you make a new list from another iterable as a reference. It collapses for loops for building lists into a single line

e.g.

nums = [12, 8, 21, 3, 16]

new_nums = [num + 1 for num in nums]

print(new_nums)

[13, 9, 22, 4, 17]

The for loop equivalent would be:

new_nums = []

for num in nums:

new_nums.append(num + 1)

Question 38

Make the following matrix using nested list comprehension:

matrix = [[0, 1, 2, 3, 4],

[0, 1, 2, 3, 4],

[0, 1, 2, 3, 4],

[0, 1, 2, 3, 4],]

Answer 38

matrix = [[col for col in range(5)] for row in range(5)]

Note that the inner list comprehension creates each list of 0-4.

Question 39

What is % called and what does it do?

Answer 39

Modulo operator

Yields the remainder from the division of the first argument by the second

Question 40

dict comprehension syntax

Answer 40

• dictname* = {key: expression for iterator variable in iterable}
  e. g.

pos_neg = {num: -num for num in range(9)}

Question 41

Use an if-else statement on the output expression of a list comprehension

Answer 41

newlist = [output if condition else altoutput for iterator in iterable]

Question 42

How is a generator different from a list comprehension?

How is it similar?

Answer 42

Differences:

• Does not construct a list
• Does not store the list in memory

Similarities:

• It’s an object we can iterate over
• We can use the same syntax as list comprehension

i. e.
* generatorname* = (output expression for iterator variable in iterable)

Question 43

What is a generator function? (4 facts)

Answer 43

• Produces generator objects when called. Note that can iterate over generator objects.
• Defined like a regular function - def
• Yields a sequence of values instead of returning a single value
• Generates a value with “yield” keyword (instead of “return”)

Example of a generator function:

def num_sequence(n):

”"”Generate values from 0 to n.”””

i=0

while i

yield i

i += 1

Question 44

How do you return the first n rows of a DataFrame?

Answer 44

dataframename.head(n)

n = number of rows to return (default is 5)