Two things you'll do constantly in almost any Python program are working with text and working with files - cleaning up user input, formatting output, or reading and writing data to disk.
Part 1: String Methods
Strings in Python come with a large set of built-in methods for inspecting, cleaning, and transforming text.
None of these methods change the original string - strings are immutable, so every method returns a new string.
Changing case
name = "Mary Wanjiku"
print(name.upper()) # MARY WANJIKU
print(name.lower()) # mary wanjiku
print(name.title()) # Mary Wanjiku
print(name.capitalize()) # Mary wanjiku
Removing whitespace
.strip() removes leading and trailing whitespace - useful when cleaning up user input or data read from a file.
raw_input = " mary@example.com "
cleaned = raw_input.strip()
print(f"'{cleaned}'") # 'mary@example.com'
print(" hello".lstrip()) # "hello" (left only)
print("hello ".rstrip()) # "hello" (right only)
Searching and checking content
text = "Data Analyst - Nairobi"
print(text.startswith("Data")) # True
print(text.endswith("Nairobi")) # True
print("Analyst" in text) # True
print(text.find("Nairobi")) # 14 (index where it starts)
print(text.find("Manager")) # -1 (not found)
Splitting and joining
.split() breaks a string into a list; .join() does the reverse.
csv_row = "Mary,Data Analyst,Nairobi"
fields = csv_row.split(",")
print(fields) # ['Mary', 'Data Analyst', 'Nairobi']
words = ["Python", "is", "fun"]
sentence = " ".join(words)
print(sentence) # Python is fun
Checking the content type of a string
Python has several methods that check what kind of characters a string contains, returning True or False. These are especially useful for validating input.
print("12345".isdigit()) # True -> all characters are digits
print("Hello".isdigit()) # False
print("Hello".isalpha()) # True -> all characters are letters
print("Hello123".isalpha()) # False -> contains digits
print("Hello123".isalnum()) # True -> all characters are letters or digits
print("Hello 123".isalnum()) # False -> contains a space
print(" ".isspace()) # True -> all characters are whitespace
print(" a ".isspace()) # False -> contains a non-space character
A practical use for these is validating simple user input before processing it:
user_input = "A1234"
if user_input.isalnum():
print("Valid: letters and numbers only")
else:
print("Invalid: contains other characters")
Counting occurrences
.count() tells you how many times a substring appears in a string:
text = "data analyst, data engineer, data scientist"
print(text.count("data")) # 3
print(text.count("engineer")) # 1
Replacing text
message = "Hello World"
print(message.replace("World", "Python")) # Hello Python
Formatting strings
f-strings are the modern, readable way to build strings that include variable values:
name = "Mary"
role = "Data Analyst"
print(f"{name} works as a {role}.") # Mary works as a Data Analyst.
price = 1234.5
print(f"Total: ${price:.2f}") # Total: $1234.50
Practical example: cleaning a list of raw entries
raw_entries = [" MARY ", "james ", " Ali", "GRACE"]
cleaned_entries = [entry.strip().title() for entry in raw_entries]
print(cleaned_entries) # ['Mary', 'James', 'Ali', 'Grace']
This kind of cleanup — stripping whitespace and standardizing case — comes up constantly when working with messy, real-world text data.
Part 2: File Handling
File handling lets your program read from and write to files on disk, so data can persist beyond a single run of the program.
Opening a file
The open() function opens a file and returns a file object. The most common modes are:
| Mode | Meaning |
|---|---|
"r" |
Read (default) — file must exist |
"w" |
Write — creates the file, overwrites if it exists |
"a" |
Append — adds to the end of the file |
"r+" |
Read and write |
Using with to handle files safely
The recommended way to work with files is the with statement, because it automatically closes the file for you, even if an error occurs partway through:
with open("notes.txt", "w") as file:
file.write("This is my first line.\n")
file.write("This is my second line.\n")
Once the with block ends, the file is closed automatically — no need to call file.close() yourself.
Reading a file
with open("notes.txt", "r") as file:
content = file.read()
print(content)
# This is my first line.
# This is my second line.
Reading line by line
For larger files, reading line by line is more memory-efficient than loading the whole file at once:
with open("notes.txt", "r") as file:
for line in file:
print(line.strip()) # .strip() removes the trailing newline
You can also get all lines as a list with .readlines():
with open("notes.txt", "r") as file:
lines = file.readlines()
print(lines) # ['This is my first line.\n', 'This is my second line.\n']