Opening and Reading Files¶

Programs often need to read data stored in files.

Python provides built-in tools for opening and reading files.

Typical tasks include:

• reading configuration files
• loading datasets
• processing logs
• reading user input stored in files

flowchart TD
    A[File on disk]
    A --> B[open()]
    B --> C[file object]
    C --> D[read operations]
````

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## 1. Opening a File

Files are opened using the `open()` function.

```python
f = open("data.txt")

This returns a file object representing the open file.

The default mode is read mode.

2. Reading the Entire File¶

f = open("data.txt")

text = f.read()
print(text)

f.close()

read() loads the entire file contents into a string.

3. Reading Line by Line¶

Files can also be processed line by line.

f = open("data.txt")

for line in f:
    print(line)

f.close()

This approach is useful for large files.

4. read(), readline(), readlines()¶

Example:

f = open("data.txt")

print(f.readline())
print(f.readline())

f.close()

5. File Closing¶

Files should normally be closed after use.

f.close()

Closing ensures resources are released and data is written properly.

Later sections introduce automatic closing using context managers.

6. Worked Example¶

f = open("numbers.txt")

for line in f:
    print(int(line))

f.close()

This example reads numbers from a file and prints them.

7. Common Pitfalls¶

Forgetting to close files¶

Unclosed files may cause resource problems.

Reading extremely large files with read()¶

This loads the entire file into memory.

Assuming files always exist¶

Attempting to open a missing file raises an exception.

8. Summary¶

Key ideas:

• files are opened with open()
• reading operations use file objects
• files can be read entirely or line by line
• files should normally be closed after use

File reading is the first step in processing external data sources.