Python Top Codes

Python Code to Validate an Email Address

Many programs ask for an email address, like a sign up or a contact form. Before saving it, you should check that it looks like a real email. In Python this is easy with the re module (regular expressions).

Python function to validate an email

import re

pattern = r"^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$"

def is_valid_email(email):
    return re.match(pattern, email) is not None

tests = [
    "robin@example.com",
    "robin@example",
    "robin.kumar+test@mail.co.in",
    "robin@@example.com",
    "@example.com",
]

for email in tests:
    print(email, "->", is_valid_email(email))

Output:

How the pattern works

Related: Python Modules, Python Strings.

Python Code to Read and Write a CSV File

A CSV (comma separated values) file stores table data as plain text, one row per line. It can be opened in Excel, and Python has a built-in csv module to work with it.

Read a CSV file

Suppose we have a file named students.csv with this content:

name,marks
Robin,85
Maria,92
John,78

DictReader reads every row as a dictionary, using the first line as the keys.

import csv

total = 0
count = 0

with open("students.csv", newline="") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["name"], row["marks"])
        total += int(row["marks"])
        count += 1

print("Average:", total / count)

Output:

Values from a CSV file are always strings, so we used int() for the marks.

Write a CSV file

import csv

rows = [
    ["name", "marks"],
    ["Robin", 85],
    ["Maria", 92],
]

with open("result.csv", "w", newline="") as f:
    writer = csv.writer(f)
    writer.writerows(rows)

print("File saved")

Output:

The new file result.csv will have this content:

name,marks
Robin,85
Maria,92

Related: Python File Handling.

Python Code to Work with JSON Data

JSON is a text format used by websites and APIs to send data. It looks like a Python dictionary. Python has a built-in json module to convert between JSON text and Python objects.

JSON string to Python (json.loads)

import json

data = '{"name": "Robin", "age": 25, "skills": ["Python", "SQL"]}'

user = json.loads(data)

print(user["name"])
print(user["skills"][0])
print(type(user))

Output:

Python to JSON string (json.dumps)

import json

user = {"name": "Robin", "age": 25, "skills": ["Python", "SQL"]}

print(json.dumps(user))

Output:

Add indent=2 to json.dumps() to get a nicely formatted text with line breaks.

Save to and load from a file

import json

user = {"name": "Robin", "age": 25}

with open("user.json", "w") as f:
    json.dump(user, f)

with open("user.json") as f:
    loaded = json.load(f)

print(loaded["age"])

Output:

Related: Python Dictionary and Python File Handling.

Python Code to Count Word Frequency in Text

Counting how often each word appears is a common job, for example to find the most used words in an article. Python makes it short with collections.Counter.

Count words with Counter

from collections import Counter

text = "to be or not to be that is the question to be"

words = text.split()
counts = Counter(words)

print(counts)
print(counts.most_common(2))
print(counts["be"])

Output:

The same without Counter

A normal dictionary also works. This is how Counter works inside.

text = "to be or not to be"
counts = {}

for word in text.split():
    counts[word] = counts.get(word, 0) + 1

print(counts)

Output:

Related: Python Dictionary and Python Strings.

Python Code to Flatten a Nested List

A nested list is a list that has other lists inside it. Flattening means making one simple list with all the values. The list can be nested to any depth, so a recursive function is the best way.

Python function to flatten a list

def flatten(items):
    result = []
    for item in items:
        if isinstance(item, list):
            result.extend(flatten(item))
        else:
            result.append(item)
    return result

nested = [1, [2, 3], [4, [5, 6]], 7]
print(flatten(nested))

Output:

How it works

• The function goes through each item of the list.
• If the item is also a list (isinstance(item, list)), the function calls itself for it and adds the flat result with extend().
• Otherwise, the item is a plain value and is added with append().

For a list that is only one level deep, a list comprehension is enough:

matrix = [[1, 2], [3, 4], [5]]
flat = [x for row in matrix for x in row]
print(flat)

Output:

Related: Python Lists and Python Functions (recursion).

Python Code to Generate a Random Password

A good password is long and random. In Python you can generate one with the secrets module, which is made for passwords and tokens.

Python function to generate a password

import secrets
import string

def generate_password(length=12):
    chars = string.ascii_letters + string.digits + "!@#$%"
    return "".join(secrets.choice(chars) for _ in range(length))

print(generate_password())
print(generate_password(16))

Sample output (the password is random, so yours will be different):

How it works

• string.ascii_letters has all upper and lower case letters, and string.digits has 0 to 9.
• secrets.choice() picks one random character from the text.
• The generator expression picks length characters, and "".join() joins them into a string.

Related: Python Modules and Python Strings.