> ## Documentation Index > Fetch the complete documentation index at: https://mintlify.com/python/cpython/llms.txt > Use this file to discover all available pages before exploring further. # Input and Output > Learn about formatting output and reading/writing files in Python Python provides several ways to present output and read input. This guide covers formatted output, file operations, and JSON serialization. ## Fancier Output Formatting There are three main ways to format output: 1. **f-strings**: Formatted string literals 2. **str.format()**: The string format method 3. **Manual formatting**: String slicing and concatenation ### Formatted String Literals (f-strings) Prefix strings with `f` or `F` and include expressions in `{}`: ```python theme={null} >>> year = 2016 >>> event = 'Referendum' >>> f'Results of the {year} {event}' 'Results of the 2016 Referendum' ``` **Format specifiers:** ```python theme={null} >>> import math >>> print(f'The value of pi is approximately {math.pi:.3f}.') The value of pi is approximately 3.142. ``` **Column alignment:** ```python theme={null} >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678} >>> for name, phone in table.items(): ... print(f'{name:10} ==> {phone:10d}') ... Sjoerd ==> 4127 Jack ==> 4098 Dcab ==> 7678 ``` **Conversion modifiers:** ```python theme={null} >>> animals = 'eels' >>> print(f'My hovercraft is full of {animals}.') My hovercraft is full of eels. >>> print(f'My hovercraft is full of {animals!r}.') My hovercraft is full of 'eels'. ``` **Self-documenting expressions:** ```python theme={null} >>> bugs = 'roaches' >>> count = 13 >>> area = 'living room' >>> print(f'Debugging {bugs=} {count=} {area=}') Debugging bugs='roaches' count=13 area='living room' ``` ### The str.format() Method Basic usage: ```python theme={null} >>> print('We are the {} who say "{}!"'.format('knights', 'Ni')) We are the knights who say "Ni!" ``` **Positional arguments:** ```python theme={null} >>> print('{0} and {1}'.format('spam', 'eggs')) spam and eggs >>> print('{1} and {0}'.format('spam', 'eggs')) eggs and spam ``` **Keyword arguments:** ```python theme={null} >>> print('This {food} is {adjective}.'.format( ... food='spam', adjective='absolutely horrible')) This spam is absolutely horrible. ``` **Accessing dictionary values:** ```python theme={null} >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678} >>> print('Jack: {0[Jack]:d}; Sjoerd: {0[Sjoerd]:d}; ' ... 'Dcab: {0[Dcab]:d}'.format(table)) Jack: 4098; Sjoerd: 4127; Dcab: 8637678 ``` Or unpack with `**`: ```python theme={null} >>> print('Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table)) Jack: 4098; Sjoerd: 4127; Dcab: 8637678 ``` **Aligned columns:** ```python theme={null} >>> for x in range(1, 11): ... print('{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x)) ... 1 1 1 2 4 8 3 9 27 4 16 64 5 25 125 6 36 216 7 49 343 8 64 512 9 81 729 10 100 1000 ``` ### Manual String Formatting String methods for formatting: ```python theme={null} >>> for x in range(1, 11): ... print(repr(x).rjust(2), repr(x*x).rjust(3), end=' ') ... print(repr(x*x*x).rjust(4)) ... 1 1 1 2 4 8 3 9 27 ``` **Zero-padding:** ```python theme={null} >>> '12'.zfill(5) '00012' >>> '-3.14'.zfill(7) '-003.14' >>> '3.14159265359'.zfill(5) '3.14159265359' ``` ### Old String Formatting The `%` operator can also format strings: ```python theme={null} >>> import math >>> print('The value of pi is approximately %5.3f.' % math.pi) The value of pi is approximately 3.142. ``` While `%` formatting still works, f-strings and `str.format()` are more powerful and easier to read. ## Reading and Writing Files Use `open()` to work with files: ```python theme={null} >>> f = open('workfile', 'w', encoding="utf-8") ``` ### File Modes | Mode | Description | | ------ | --------------------------------- | | `'r'` | Read only (default) | | `'w'` | Write only (erases existing file) | | `'a'` | Append to file | | `'r+'` | Read and write | | `'b'` | Binary mode | Always specify `encoding="utf-8"` when opening text files to ensure consistent behavior across platforms. ### Using with Statements The `with` keyword ensures files are properly closed: ```python theme={null} >>> with open('workfile', encoding="utf-8") as f: ... read_data = f.read() >>> f.closed True ``` Without `with`, you must manually close files: ```python theme={null} >>> f = open('workfile', encoding="utf-8") >>> # ... work with file ... >>> f.close() ``` Failing to close files can result in data loss. Always use `with` statements or explicitly call `f.close()`. ## Methods of File Objects ### Reading Files **Read entire file:** ```python theme={null} >>> f.read() 'This is the entire file.\n' >>> f.read() # End of file reached '' ``` **Read a single line:** ```python theme={null} >>> f.readline() 'This is the first line of the file.\n' >>> f.readline() 'Second line of the file\n' >>> f.readline() # End of file '' ``` **Loop over lines (efficient):** ```python theme={null} >>> for line in f: ... print(line, end='') ... This is the first line of the file. Second line of the file ``` **Read all lines into a list:** ```python theme={null} >>> list(f) ['This is the first line of the file.\n', 'Second line of the file\n'] ``` ### Writing Files **Write a string:** ```python theme={null} >>> f.write('This is a test\n') 15 ``` `write()` returns the number of characters written. **Write other types:** ```python theme={null} >>> value = ('the answer', 42) >>> s = str(value) # convert to string >>> f.write(s) 18 ``` ### File Positioning **Get current position:** ```python theme={null} >>> f.tell() 16 ``` **Change position:** ```python theme={null} >>> f = open('workfile', 'rb+') >>> f.write(b'0123456789abcdef') 16 >>> f.seek(5) # Go to 6th byte 5 >>> f.read(1) b'5' >>> f.seek(-3, 2) # Go to 3rd byte before the end 13 >>> f.read(1) b'd' ``` **seek() parameters:** * `whence=0`: From beginning (default) * `whence=1`: From current position * `whence=2`: From end of file ## Saving Structured Data with JSON JSON (JavaScript Object Notation) is perfect for serializing Python data: ```python theme={null} >>> import json >>> x = [1, 'simple', 'list'] >>> json.dumps(x) '[1, "simple", "list"]' ``` ### Writing JSON to a File ```python theme={null} import json data = {'name': 'Alice', 'age': 30, 'city': 'New York'} with open('data.json', 'w', encoding='utf-8') as f: json.dump(data, f) ``` ### Reading JSON from a File ```python theme={null} import json with open('data.json', 'r', encoding='utf-8') as f: data = json.load(f) print(data) # {'name': 'Alice', 'age': 30, 'city': 'New York'} ``` JSON files must be encoded in UTF-8. Always use `encoding="utf-8"` when opening JSON files. ### JSON Limitations * Handles lists and dictionaries well * Arbitrary class instances require extra work * For Python-specific serialization, consider `pickle` (but it's insecure with untrusted data) **Security Note**: The `pickle` module can execute arbitrary code when deserializing data. Never unpickle data from untrusted sources. ## Complete Example Here's a complete example combining file I/O and JSON: ```python theme={null} import json # Data to save students = [ {'name': 'Alice', 'grade': 85}, {'name': 'Bob', 'grade': 92}, {'name': 'Charlie', 'grade': 78} ] # Write to file with open('students.json', 'w', encoding='utf-8') as f: json.dump(students, f, indent=2) print("Data written to file.") # Read from file with open('students.json', 'r', encoding='utf-8') as f: loaded_students = json.load(f) # Display the data for student in loaded_students: print(f"{student['name']}: {student['grade']}") ``` ## Next Steps You now understand file I/O and data serialization. Next, learn how to handle [Errors and Exceptions](/tutorial/errors) gracefully in your programs.