PythonFile IoIntermediate

json.load / json.dump

Functions from the `json` module for reading (deserializing) and writing (serializing) JSON data directly to/from file-like objects.

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json.load(fp)
json.dump(obj, fp, *, skipkeys=False, ensure_ascii=True, check_circular=True, allow_nan=True, cls=None, indent=None, separators=None, default=None, sort_keys=False)
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What it does

Overview

Functions from the `json` module for reading (deserializing) and writing (serializing) JSON data directly to/from file-like objects.

The `json` module provides a convenient way to work with JSON (JavaScript Object Notation) data, which is a lightweight data-interchange format. `json.dump(obj, fp)` serializes a Python object (`obj`) into a JSON formatted string and writes it to a file-like object (`fp`). This is the file-based counterpart to `json.dumps()`, which returns a JSON string. Key parameters for `json.dump()` include `indent` for pretty-printing (e.g., `indent=4`), and `eys=True` to output dictionaries with keys sorted alphabetically. `json.load(fp)` deserializes a JSON document from a file-like object (`fp`) and returns the corresponding Python object. This is the file-based counterpart to `json.loads()`, which parses a JSON string. Both `dump` and `load` handle the file I/O operations directly, making them ideal for persistent storage of Python data structures in a human-readable and interoperable format. When working with JSON files, it's crucial to open them in text mode (`'w'` for writing, `'r'` for reading) and use a `with` statement to ensure proper file handling and resource management. Error handling, such as catching `json.JSONDecodeError` for malformed JSON, is also a best practice.

Quick reference

Syntax

json.load(fp)
json.dump(obj, fp, *, skipkeys=False, ensure_ascii=True, check_circular=True, allow_nan=True, cls=None, indent=None, separators=None, default=None, sort_keys=False)

Inputs

Parameters

objAny · The Python object to be serialized to JSON.
fpfile-like object · A file-like object (e.g., returned by `open()`) to which JSON data is written or from which it is read.
indent (optional)int | None · If provided, JSON output will be pretty-printed with the specified indent level.
sort_keys (optional)bool · If True, dictionary keys will be sorted in the output.

See it in practice

Examples

1

Dumping a Python dictionary to a JSON file

import json

data = {
    'name': 'Alice',
    'age': 30,
    'isStudent': False,
    'courses': ['Math', 'Science']
}

with open('data.json', 'w') as f:
    json.dump(data, f, indent=4)

# Verify content (optional, for demonstration)
with open('data.json', 'r') as f:
    print(f.read())
Output:
{ "name": "Alice", "age": 30, "isStudent": false, "courses": [ "Math", "Science" ] }

This example serializes a Python dictionary `data` into a JSON file named `data.json`. The `indent=4` argument ensures the output is pretty-printed with 4 spaces for readability.

2

Loading JSON data from a file into a Python dictionary

import json

# First, create a sample JSON file
with open('sample.json', 'w') as f:
    f.write('{"city": "New York", "population": 8400000}')

# Now, load the data
with open('sample.json', 'r') as f:
    loaded_data = json.load(f)
    print(f"Loaded data: {loaded_data}")
    print(f"Type of loaded data: {type(loaded_data)}")
Output:
Loaded data: {'city': 'New York', 'population': 8400000} Type of loaded data: <class 'dict'>

This demonstrates `json.load()` reading a JSON formatted string from `sample.json` and converting it back into a Python dictionary. The `json` module automatically maps JSON types to their corresponding Python types.

3

Dumping with sorted keys and loading back

import json

data = {'b': 2, 'a': 1, 'c': 3}

with open('sorted_data.json', 'w') as f:
    json.dump(data, f, indent=2, sort_keys=True)

with open('sorted_data.json', 'r') as f:
    loaded_sorted_data = json.load(f)
    print(f"Dumped and loaded data: {loaded_sorted_data}")
Output:
Dumped and loaded data: {'a': 1, 'b': 2, 'c': 3}

The `eys=True` argument ensures that the keys in the JSON output are sorted alphabetically. When loaded back, Python dictionaries (from 3.7+) preserve insertion order, so the loaded dictionary will also have its keys sorted.

Debug faster

Common Errors

1

json.JSONDecodeError

Cause: Attempting to load a file that is not valid JSON, or contains syntax errors.

Fix: Ensure the input file contains well-formed JSON. Use online validators or carefully inspect the JSON structure. Implement `try-except json.JSONDecodeError` blocks for robust parsing.

import json

with open('malformed.json', 'w') as f:
    f.write('{"key": "value", "another_key": }') # Missing value

# This will raise a json.JSONDecodeError
# with open('malformed.json', 'r') as f:
#     json.load(f)

Runtime support

Compatibility

AnyN/AAll Python versions

Source: Python Docs

Common questions

Frequently Asked Questions

Functions from the `json` module for reading (deserializing) and writing (serializing) JSON data directly to/from file-like objects.

obj: The Python object to be serialized to JSON. fp: A file-like object (e.g., returned by `open()`) to which JSON data is written or from which it is read. indent: If provided, JSON output will be pretty-printed with the specified indent level. eys: If True, dictionary keys will be sorted in the output.

json.JSONDecodeError: Ensure the input file contains well-formed JSON. Use online validators or carefully inspect the JSON structure. Implement `try-except json.JSONDecodeError` blocks for robust parsing.