Learn Python Coding

reversed() in Python - what supports it and what doesn't

The function reversed() is built-in in Python, but it doesn't work with all data types

✓ Lists - it works
reversed([1, 2, 3]) returns an iterator
list(reversed([1, 2, 3])) → [3, 2, 1]

✓ Tuples - it also works
reversed((1, 2, 3)) can be easily iterated

✗ Sets - not supported
reversed({1, 2, 3}) → TypeError
Why? Sets don't have a fixed order, so they can't be "reversed"

If you need to reverse a set:
list(reversed(list({1, 2, 3})))

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⚡️ Python code that works, but does extra work 100 times over

This Python code looks normal.
It works.
It passes the tests.
But it does extra work dozens, and sometimes hundreds of times.

The most common reason is that you accidentally turn a linear algorithm into a quadratic one.

A typical scenario:
- there's a list
- inside the loop, you repeatedly do in, count, index
- everything works quickly with small data
- on real data, the application starts to "slow down for no reason"

The problem is that:
- list is O(n) for searching
- searching inside the loop = O(n²)
- Python honestly does the work you asked it to do

Pros don't think about "whether it works or not", but how many extra operations are being performed.

The correct approach:
- if you need membership checks, use set
- if you're counting elements, use dict or Counter
- if the data doesn't change, pre-calculate it once

This technique is one of the most common sources of hidden performance bugs in Python code.

# ❌ Bad: O(n²)
users = ["alice", "bob", "carol", "dave"]

for u in users:
    if u in users:   # full list traversal every time
        process(u)


# ✅ Good: O(n)
users = ["alice", "bob", "carol", "dave"]
users_set = set(users)

for u in users:
    if u in users_set:
        process(u)

A bit of Python basics. Day 7. Counting the number of occurrences of an element

If you need to find out how many times an element appears in an iterable collection, you can use the Counter class from the collections module. Counter() returns a dictionary with the number of times each element appears in the sequence. Let's say we want to find out how many times the name Peter appears in the following list. We can use Counter(). See below:

from collections import Counter

list1 = ['John', 'Kelly', 'Peter', 'Moses', 'Peter']

count_peter = Counter(list1).get("Peter")

print(f'The name "Peter" appears in the list '
      f'{count_peter} times.')

Output:

The name "Peter" appears in the list 2 times.

Another way to do this is with a regular for loop. We create a count variable and increase it by 1 each time we find the name Peter in the sequence. This is a naive approach. See below:

list1 = ['John', 'Kelly', 'Peter', 'Moses', 'Peter']
# Create a count variable
count = 0
for name in list1:
    if name == 'Peter':
        count +=1
print(f'The name "Peter" appears in the list'
      f' {count} times.')

Output:

The name "Peter" appears in the list 2 times.

Lists and other iterable data structures in Python have a built-in count() method, which allows us to count the number of occurrences of a specific element. We can use count() to count how many times Peter appears in the list.

list1 = ['John', 'Kelly', 'Peter', 'Moses', 'Peter']

print(f'The name "Peter" appears in the list '
      f'{list1.count("Peter")} times.')

Output:

The name "Peter" appears in the list 2 times.

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A bit of Python basics. Day 6 - Exchanging variable values

In Python, you can swap variables after they have already been assigned objects. Below, we first assign 20 to the variable x and 30 to the variable y, and then swap them: x becomes 30, and y becomes 20. This method is called tuple packing/unpacking.

x, y = 20, 30
x, y = y, x

print('x is: ', x)
print('y is: ', y)

x is 30
y is 20

You can also use the XOR (exclusive or) operator to swap variables. This is a three-step method. In the example below, we swap the values of x and y.

x = 20
y = 30

# step one
x ^= y
# step two
y ^= x
# step three
x ^= y

print(f'x is: {x}')
print(f'y is: {y}')

x is: 30
y is: 20

You can also use arithmetic operations (addition and subtraction) to swap variables without a temporary variable. However, this method is recommended for swapping numeric data types. Here's an example:

# Use arithmetic operations
x = 5
y = 10

x = x + y
y = x - y
x = x - y

print("After swapping:")
print("x =", x)
print("y =", y)

After swapping:
x = 10
y = 5

As a result of these arithmetic operations, the values of x and y have actually been swapped. After the swap, x contains the original value of y (10), and y contains the original value of x (5).

This method of swapping variables without a temporary variable is based on the fact that when you add or subtract the value of one variable from another, you can effectively swap their values without the need for additional storage.

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A bit of Python basics. Day 4: Getting the current time and date

The code below shows how to get the current time using the datetime module. The now() method returns a datetime object representing the current date and time according to the system clock. The strftime() method formats the time for the desired output. This code shows how to use the datetime module together with the strftime() method to get a formatted time string in the format of hours, minutes, and seconds.

from datetime import datetime

time_now = datetime.now().strftime('%H:%M:%S')
print(f'Current time: {time_now}')

Current time: 17:37:28

What if we want to return today's date? We can use date from the datetime module. Below, the today() method is used:

from datetime import date

today_date = date.today()
print(today_date)

2023-09-20

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A bit of basics. Day 3: Calendar in Python

There is a built-in module in Python called calendar. We can import this module to display the calendar. There are many things you can do with the calendar.

Let's say we want to see the calendar for April 2022. We use the month class from the calendar module and pass the year and month as arguments. See below:

import calendar

month = calendar.month(2022, 4)
print(month)

There are many other things you can do with calendar. For example, you can use it to check whether a given year is a leap year or not. Let's check if 2022 is a leap year.

import calendar

month = calendar.isleap(2022)
print(month)

False

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Python tip

Adding an element to the end of a list (list.append) works in O(1), but inserting in the middle of the list is O(n) because elements need to be shifted

If you need to efficiently add elements from both ends, use collections.deque — there operations on the edges have complexity O(1)

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