README.md

File Handling and JSON in Python

This lesson convers some of the basics of file input and output using Python, as well as the popular JSON data encoding.

1. Working with open() built-in method to access a file

$ echo "hello world" > input.txt

filename = 'input.txt'

f = open(filename)
contents = f.read()
f.close()

print(contents)

1. What is going on here?

   1. First we create a variable to hold the filename as a string
   2. Next we pass that variable to the built-in Python open function
   3. We read out the contents of the file, and then close the file
   4. Finally we print out the contents

2. Simplifying our code

   The close step in the program is interesting. Almost every file open should be followed by a corresponding file close. Python provides us with a way to make this process slightly easier. The program below is completely functionally equivalent:

filename = 'input.txt'

with open(filename) as f:
    contents = f.read()

print(contents)

1. Best practice - use open with the code pattern above, with...as. This is called a 'context manager', and will ensure that the file is closed even if your code encounters an exception.

2. The colon on line 3 indicates the beginning of a code block (also used for if and else conditions). The file f is only open until we reach the end of the block.

3. Let's take a look at the docs for [open][https://docs.python.org/2/library/functions.html#open]

4. So, what modes can we open a file in?

   1. r - read mode. Read a file from the first byte. Cannot write.
   2. w - delete the whole file and start writing from the first byte.
   3. a - append a file starting at the last byte.
   4. rw - read and write. be careful! if you read up to a point you can overwrite from there. This is challenging to use and you never really need it.
   5. b - the Windows platform considers text and binary files to be different, so we need to specify this if we are writing 'raw' bytes to a file on a Windows computer

Lets try using a file mode:

filename = 'output.txt'

with open(filename, 'w') as f:
	f.write('The world is talking back to you.\n')

3. Review JSON format and discuss Python types

JSON (JavaScript Object Notation) is a file format which can be used to store and communicate data between computer systems, while still being (arguably) readable by humans. It's commonly used as a format for storing data in files and for as a 'response' format for web APIs.

A JSON document describing some of the metadata about Noisebridge could be written as:

{
    "address": "2169 Mission Street, San Francisco, CA",
    "name": "Noisebridge",
    "open": true,
    "members": ["a", "b", "c", ... ]
}

Note that JSON is a little stricter than Python with regards to quotation marks and trailing commas; we always have to use double quotes for fields/values, and we are not allowed trailing commas in lists or objects.

The full JSON format is specified at json.org as a railroad diagram.

Let's use a built-in Python tool to pretty-print some JSON data so that it's more readable for us.

{"url":"www.example.com","metadata":{"crawled_at":"2018-01-15","title":"Example Domain"}}

python -m json.tool example.json`

Python Data Types can be encoded to to JSON (and back!)

This is called encoding and decoding. This allows complex data structures to exist easily as flat strings. It makes it easy to transport all this structure.

4. Lets convert some JSON!

Two key actions are encode and decode.

In the Python json module, these will be available as dump/load and dumps/loads

Encoding and decoding is done by the json module. There are more details here.

Example: Using json.loads() and json.dumps()

>>> import json
>>> help(json)
>>>
>>> my_json_string = u'{"message":"hello world"}'
>>> json.loads(my_json_string)
{u'message': u'hello world'}
>>>
>>> my_dict = json.loads(my_json_string)
>>> my_dict
{u'message': u'hello world'}
>>>
>>> json.dumps(my_dict)
'{"message": "hello world"}'
>>>

Exercise: Using json.load() and json.dump()

"""
import json

data = {"message": "hello world"}

filename = 'data.json'

# write the data into the file data.json
# close the data.json file

# read the data back from data.json
# print the newly-read data and then close data.json