Python provides several ways to present output and read input. This guide covers formatted output, file operations, and JSON serialization.
There are three main ways to format output:
- f-strings: Formatted string literals
- str.format(): The string format method
- Manual formatting: String slicing and concatenation
Prefix strings with f or F and include expressions in {}:
>>> year = 2016
>>> event = 'Referendum'
>>> f'Results of the {year} {event}'
'Results of the 2016 Referendum'
>>> import math
>>> print(f'The value of pi is approximately {math.pi:.3f}.')
The value of pi is approximately 3.142.
>>> 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
>>> 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'.
>>> bugs = 'roaches'
>>> count = 13
>>> area = 'living room'
>>> print(f'Debugging {bugs=} {count=} {area=}')
Debugging bugs='roaches' count=13 area='living room'
>>> print('We are the {} who say "{}!"'.format('knights', 'Ni'))
We are the knights who say "Ni!"
>>> print('{0} and {1}'.format('spam', 'eggs'))
spam and eggs
>>> print('{1} and {0}'.format('spam', 'eggs'))
eggs and spam
>>> print('This {food} is {adjective}.'.format(
... food='spam', adjective='absolutely horrible'))
This spam is absolutely horrible.
>>> 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
**:
>>> print('Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table))
Jack: 4098; Sjoerd: 4127; Dcab: 8637678
>>> 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
>>> 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
>>> '12'.zfill(5)
'00012'
>>> '-3.14'.zfill(7)
'-003.14'
>>> '3.14159265359'.zfill(5)
'3.14159265359'
% operator can also format strings:
>>> 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:
>>> 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:
>>> with open('workfile', encoding="utf-8") as f:
... read_data = f.read()
>>> f.closed
True
with, you must manually close files:
>>> 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:
>>> f.read()
'This is the entire file.\n'
>>> f.read() # End of file reached
''
>>> f.readline()
'This is the first line of the file.\n'
>>> f.readline()
'Second line of the file\n'
>>> f.readline() # End of file
''
>>> for line in f:
... print(line, end='')
...
This is the first line of the file.
Second line of the file
>>> list(f)
['This is the first line of the file.\n', 'Second line of the file\n']
Writing Files
Write a string:
>>> f.write('This is a test\n')
15
write() returns the number of characters written.
>>> value = ('the answer', 42)
>>> s = str(value) # convert to string
>>> f.write(s)
18
File Positioning
Get current position:
Change position:
>>> 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'
whence=0: From beginning (default)whence=1: From current positionwhence=2: From end of file
Saving Structured Data with JSON
JSON (JavaScript Object Notation) is perfect for serializing Python data:
>>> import json
>>> x = [1, 'simple', 'list']
>>> json.dumps(x)
'[1, "simple", "list"]'
Writing JSON to a File
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
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:
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 gracefully in your programs. 