# Python Fileinput Module

The `fileinput` module implements a helper class and functions to quickly write a loop over standard input or a list of files. This is useful for writing filter scripts that process input streams line by line.

## Importing the Module

```python
import fileinput
```

## Basic Usage

The typical use case is to iterate over the lines of all files listed in `sys.argv[1:]`, defaulting to `sys.stdin` if the list is empty.

```python
import fileinput

for line in fileinput.input():
    print(f"Processing: {line}", end='')
```

You can run this script like: `python script.py file1.txt file2.txt`.

## Specifying Files

You can explicitly pass a list of filenames to `fileinput.input()`.

```python
import fileinput

files = ['data1.txt', 'data2.txt']
with fileinput.input(files=files) as f:
    for line in f:
        print(line, end='')
```

## In-Place Editing

The module supports in-place editing of files. When `inplace=True` is passed to `fileinput.input()`, standard output is redirected to the input file.

```python
import fileinput

# Adds line numbers to a file, modifying it in-place
with fileinput.input(files=('test.txt',), inplace=True, backup='.bak') as f:
    for line in f:
        print(f"{f.lineno()} {line}", end='')
```

## File Metadata

You can access information about the current file and line being processed.

*   `fileinput.filename()`: Name of the file currently being read.
*   `fileinput.lineno()`: Cumulative line number.
*   `fileinput.filelineno()`: Line number in the current file.
*   `fileinput.isfirstline()`: True if the line read is the first line of its file.

```python
import fileinput

for line in fileinput.input():
    if fileinput.isfirstline():
        print(f"--- Reading {fileinput.filename()} ---")
    print(f"Line {fileinput.filelineno()}: {line}", end='')
```

[[programming/python/python]]