Writing to and Reading from Files

File Handling and Exception Handling

Learning Outcome

Connect try-except with the with statement.

Apply the right read or write method to real tasks.

Recognize why catching specific errors matters.

Identify the common errors file operations raise.

Understand how Python reads and writes file data.

The Courier

A courier delivers parcels,

Sometimes a whole sack, sometimes one letter.
Some addresses don’t exist; some buildings are locked.

A panicked courier abandons the van. A trained one returns the parcel with a note and keeps going.

Expected Answer: The panicked courier. Without error handling, one failed operation crashes the whole program.

Exactly! But before training the courier, let’s look at the normal delivery work: reading and writing.

A program opens “marks.csv”, but the file name has a typo. It stops instantly and shows a technical error. Which courier is this?

Reading Data from a File

read() returns the whole file as one string; read(100) reads only 100 characters.

readline() reads one line, including its \n, and returns an empty string at the end.

Reading Data from a File

readlines() returns all lines as a list, each still ending in \n.

for line in f: reads one line at a time and holds only that line in memory.

Code: for line in f:

Why it matters: Use read() for small files. For large files, loop line by line so a 5 GB file never loads into memory at once.

Clean Data and the File Pointer

Lines read from a file carry a trailing \n. line.strip() removes it along with extra spaces.
Every read method moves an invisible file pointer forward.
Once read() reaches the end, calling it again returns an empty string, because nothing is left.
f.seek(0) moves the pointer back to the start so you can read again.

Code: line.strip()

Writing Data to a File

write() writes one string and returns the number of characters written.
writelines() writes a list of strings one after another.
Neither adds newlines, so put \n yourself, e.g. f.write("Rahul\n").
Both accept only strings. f.write(95) raises a TypeError; use str(95) or "\n".join(items).

Core Mechanism: What Can Go Wrong?

File operations depend on things outside the program’s control: folders, permissions, and file contents.