ML / Python Flashcards
Create array from array without reference [python]
a = b[:]
how to declare a list and map? [python]
mp = {}
lst = []How to check if the element is int or array? [python]
for v in data_list:
if isinstance(v, int):Implement decorator which returns function signature and its return value
def debug(func):
"""
:param func: function
"""
def wrapper(*args, **kwargs):
output = func(*args, **kwargs)
return f'{func.\_\_name\_\_}{args} was called and returned {output}'
return wrapper
@debug def add(a, b):
How to remove the element from the list with the index? [python]
del lst[i]
how to inherit class in Python?
class Shuttle(Rocket):
how to call super from child class?
super().__init__(name, mission)
What Is Python?
Interpreted high-level object-oriented dynamically-typed scripting language.
python standard data types
numbers, strings, sets, lists, tuples, and dictionaries
Can we update string in Python?
Strings are immutable. Once they are created, they cannot be changed e.g.
a = ‘me’
Updating it will fail:
a[1]=’y’
What if we create variable in for-loop or if ??? [python]
if - we can use it after if
for-loop - we can’t use it
if is_python_awesome:
test_scope = “Python is awesome”
print(test_scope) it is ok
how to use global variable in Python?
TestMode = True def some_function(): global TestMode TestMode = False some_function() print(TestMode)
how to get type of variable in Python?
type(‘farhad’)
–> Returns
what is ‘divmod’ in python?
print(divmod(10,3)) #it will print 3 and 1 as 3*3 = 9 +1 = 10
repeat string in python
‘A’*3 will repeat A three times: AAA
how to reverse string in python?
x = 'abc' x = x[::-1]
how to find index of second ‘a’ in a string? [python]
name = 'farhad'
index = name.find('a', 2) # finds index of second aRegex uses in python for working with strings
split(): splits a string into a list via regex
sub(): replaces matched string via regex
subn(): replaces matched string via regex and returns number of replacements
how to cast variable to type in python?
str(x): To string
int(x): To integer
float(x): To floats
how to remove element from the set in Python?
set. remove(item) — removes item from the set and raises error if it is not present
set. discard(item) — removes item from the set if it is present
how to get any element from the set in python?
set.pop() — returns any item from the set, raises KeyError if the set is empty
operations with sets in python?
a = {1,2,3}
b = {3,4,5}
c = a.intersection(b)
c = a.difference(b)
c = a.union(b)what is pickling in python?
Converting an object into a string and dumping the string into a binary file is known as pickling. The reverse is known as unpickling.
If your function can take in any number of arguments - what to do in python?
then add a * in front of the parameter name: def myfunc(*arguments): return a
what is **arguments in python?
def test(*args, **kargs):
print(args)
print(kargs)
print(args[0])
print(kargs.get('a'))
alpha = ‘alpha’
beta = ‘beta’
test(alpha, beta, a=1, b=2)
(3, 1)
(‘alpha’, ‘beta’)
{‘a’: 1, ‘b’: 2}
alpha
1
It allows you to pass a varying number of keyword arguments to a function.
You can also pass in dictionary values as keyword arguments.
how to use lambda in Python?
variable = lambda arguments: expression
difference == vs is in python?
If we use == then it will check whether the two arguments contain the same data
If we use is then Python will check whether the two objects refer to the same object. The id() needs to be the same for both of the objects
what is PIP in python?
PIP is a Python package manager.
Use PIP to download packages: pip install package_name
how to check types in python?
if not isinstance(input, int):
Let’s assume your list contains a trillion records and you are required to count the number of even numbers from the list. It will not be optimum to load the entire list in the memory. You can instead yield each item from the list.
range(start, stop, step):
Generates numerical values that start at start, stop at stop with the provided steps. As an instance, to generate odd numbers from 1 to 9, do:
rint(list(range(1,10,2)))
what is tuples in python?
Tuples are like lists in the sense that they can store a sequence of objects. The objects, again, can be of any type.
Tuples are faster than lists.
These collections are indexed by integers.
Tuples are immutable (non-update-able)
my_tuple = tuple()
or
my_tuple = 'f','m'
or
my_tuple = ('f', 'm')
Print dictionary contents in python
for key in dictionary:
print key, dictionary[key]
get all items from dictionary in python?
dictionary.items() # returns items
#checking if a key exists in a dictionary
if ('some key' in dictionary):
#do somethingremove operations with dictionaries in python?
pop(key, default): returns value for key and deletes the item with key else returns default
popitem(): removes random item from the dictionary
merges two dictionaries in python
dictionary1.update(dictionary2)
what is zip in python
takes multiple collections and returns a new collection.
The new collection contains items where each item contains one element from each input collection.
It allows us to transverse multiple collections at the same time
name = ‘farhad’
suffix = [1,2,3,4,5,6]
zip(name, suffix)
–> returns (f,1),(a,2),(r,3),(h,4),(a,5),(d,6)
__str__ in python?
Returns stringified version of an object when we call “print”:
__cmp__
in python?
Use the __cmp__ instance function if we want to provide custom logic to compare two objects of the same instance.
It returns 1 (greater), -1 (lower) and 0 (equal) to indicate the equality of two objects.
Think of __cmp__ like Equals() method in other programming language.
does python supports multiple inheritance?
Note: Python supports multiple inheritances unlike C# class A(B,C): #A implments B and C
If you want to call parent class function then you can do: [python]
class A(B,C): #A implments B and C
super(A, self).function_name()
what about garbage collection in python?
All of the objects in Python are stored in heap space. This space is accessible to the Python interpreter. It is private.
Python has an in-built garbage collection mechanism.
Open a connection in sql and execute statement? [python]
import MySQLdb
database = MySQLdb.connect(“host”=”server”, “database-user”=”my username”, “password”=”my password”, “database-name”=”my database”)
cursor = database.cursor()
cursor. fetch(“Select * From MyTable”)
database. close()
web services - to query a rest service in python
import requests
url = ‘http://myblog.com’
response = requests.get(url).text
To Serialise and Deserialise JSON [python]
[deserialize]
import json
my_json = {"A":"1","B":"2"}
json_object = json.loads(my_json)
value_of_B = json_object["B"]
[serialize]
import json
a = “1”
json_a = json.dumps(a)
how to insert values between indexes in python?
a[2:2] = [3, 4, 5, 6]
makes from 1 2 7 8 -> 1 2 3 4 5 6 7 8
how to create a tuple with one element in python?
we can’t create a tuple with single element like (‘s’)
the correct answer is t = (‘foo’,)
what we get (1, 2, 3, 4, 5, 6, 7, 8, 9)[1::3] in python?
(2, 5, 8)
x = 5
y = -5
what we get in python from (y, x)[::-1] ?
(5, -5)
parameters in split()? python
split() takes in two parameters: sep, the delimiter string, and maxsplit, which specifies the maximum number of splits to make on the input string.
what is immutability in python strings mean?
strings can’t be changed
function ord() in python
get ASCII code of character
»> print(ord(‘f’))
102
raises an exception in case trying to set string as parameter
return Hello, my name is .’, with person inserted in place of . in python
return f’Hello, my name is {person}.’
Note the difference between np.ndarray and np.array()
The former is an actual data type, while the latter is a function to make arrays from other data structures.
How To Create a Pandas DataFrame
ata = np.array([[’’,’Col1’,’Col2’],
[‘Row1’,1,2],
[‘Row2’,3,4]])
print(pd.DataFrame(data=data[1:,1:],
index=data[1:,0],
columns=data[0,1:]))
you first select the values that are contained in the lists that start with Row1 and Row2, then you select the index or row numbers Row1 and Row2 and then the column names Col1 and Col2.
how to get information about dataframe?
print(df.shape) - width and height
print(len(df.index)) - get height
output:
(2,3)
2
how to get elements on the certain index in pandas?
A B C
0 1 2 3
1 4 5 6
2 7 8 9
# Using `iloc[]` print(df.iloc[0][0])
# Using `loc[]` print(df.loc[0]['A'])
# Using `at[]` print(df.at[0,'A'])
# Using `iat[]` print(df.iat[0,0])
difference between .loc[] and .iloc[]
.loc[] works on labels of your index. This means that if you give in loc[2], you look for the values of your DataFrame that have an index labeled 2.
.iloc[] works on the positions in your index. This means that if you give in iloc[2], you look for the values of your DataFrame that are at index ’2`.
Adding a Column to Your DataFrame
df = pd.DataFrame(data=np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]]), columns=[‘A’, ‘B’, ‘C’])
# Use `.index` df['D'] = df.index
D
0
1
2
Resetting the Index of Your DataFrame
df_reset = df.reset_index(level=0, drop=True)
remove the index name, if there is any, by executing [dataframe]
del df.index.name
Deleting a Column from Your DataFrame
df.drop(‘A’, axis=1, inplace=True)
Removing a Row from Your DataFrame
df. drop_duplicates([48], keep=’last’)
df. drop(df.index[1])
How To Iterate Over a Pandas DataFrame
for index, row in df.iterrows() :
print(row[‘A’], row[‘B’])
Output a DataFrame to CSV
import pandas as pd
df.to_csv(‘myDataFrame.csv’)
drop rows with missing values
df.dropna()
new dict with empty values from existing dict
D = dict.fromkeys(keys)
sorted list of keys from dict in python
sorted(D.keys())
value swapping in python
a, b = b, a
Create a single string from all the elements in list [Python]
” “.join(a)
Find The Most Frequent Value In A List [Python]
max(set(a), key=a.count)) or from collections import Counter cnt = Counter(a) cnt.most_common(3)
Chained function call in python
b - True
product and add - functions
(product if b else add)(5,7)
Remove duplicates from a list in python
save order / and not
items = [2,3,4,5,6]
list(set(items))
from collections import OrderedDict
list(OrderedDict.fromkeys(items).keys())
matrix product [numpy]
A - np.array()
A @ B
A.dot(B)
[pandas] how to get dataframe from dictionaries and transpose it?
return pd.DataFrame.from_dict(data, orient=’index’, columns=labels).T
what is the difference between np.array and np.asarray?
by default, np.array copies the objects
how to get 3 biggest elements of the following array? [numpy
ar = np.array([2,2,2,2,23,4,5,5,6])
how to save NumPy array in a file
np.savetxt(‘foo.csv’,arr, delimiter=’,’)
suppose you want to join train and test dataset (both are two numpy arrays train_set and test_set) into a reshaping array (resulting_ste) to do data processing on it simultaneously.
resulting_set = np.vstack([train_set, test_test])
what is supervised learning?
we are given a data set and already know what our correct output should look like, having the idea that there is a relationship between input and output
what is unsupervised learning?
allows us to approach a problem with continious predict in a what our results should look like
supervised learning - 2 types?
regression and classification
gradient descent algorithms
theta := theta - alpha * disc( J(theta(s))
what is convex function?
only one global minimum
formula of gradient descent
… check 2nd page of notes
what is feature scaling?
replace xi with xi - mi to make features have approximately zero mean (do no apply to x0)
how to choose learning rate alpha?
0.001 0.003 0.01 0.03 0.1 0.3 1 …
declare convergence if J decreases by less that 10^-3
how to improve out features?
- combine different features into one
- change the begav of the curve by making it quadratic, cubic, squares
“high bias” what is it?
underfit
“high variance”
overfit
what is regularization?
J = J + lambda * sum ( theta.^2)
Effective K-Means Algorithm
how to choose number of clusters?
Elbow method
What is PCA?
PCA is a technique used to reduce the number of dimensions in a dataset while preserving the most important information in it
What is enumerate in Python?
(0, “d”), (1, “d”) -> enumerate(list)
what is hyperparameter tuning?
the process to find the most optimal hyperparameters to avoid overfitting or underfitting.
What if we inherited 2 classes in Python and they have the same methods?
MRO - Method Resolution Order
Three (One, Two) -> so we firstly take method from Three, then One and then Two
