files-json

Review Files and JSON

Writing

Let's try writing a file. Paste the following and run it (it's buggy!):

f = open("file.txt")
f.write("line1")
f.write("line2")
f.close() # you need the parentheses, even without arguments!

The code fails because the file wasn't opened in write mode. Add "w" as a second positional arguments to open, then try again.

Does the above code actually produce two lines? Open file.txt through Jupyter and check. Try modifying the "line1" and "line2" strings to get two different lines, so that file.txt looks like this:

line1
line2

Reading

Create a file named dog.json in Jupyter. You can use the "New / Text File" menu, like this:

Copy the following:

{
    "name": "Fido",
    "age": 1
}

Paste it into dog.json, and be sure to SAVE it:

Now, back in your notebook, run this:

f = open("dog.json")
input1 = f.read()
input2 = f.read()
f.close()

What is type(input1)?

Print out input1, then print input2. Notice that input2 is the empty string? That's because the first read call consumes all the input, and there's nothing left for the second read call. To get the data again, you could re-open the file, then re-read it again.

Note that f is a file object. Calling .read is only one way to get the contents. File objects are iterators, meaning that you could also loop over f -- try it:

f = open("dog.json")
for line in f:
    print("LINE: " + line, end="")
f.close()

You can also create lists from iterators. Try that:

f = open("dog.json")
lines = list(f)
f.close()

print("GOT", len(lines), "lines")

with

It's easy to forget to close a file. Python has a special with statement that can do it automatically for you. This:

f = open("dog.json")
lines = list(f)
f.close()

Is equivalent to this (try it):

with open("dog.json") as f:
    lines = list(f)
# f is automatically closed after the with block

JSON

Your data may be in the following forms:

1. a file object
2. a string
3. a dict (or other Python data type)

json.load converts from (1) to (3). json.loads converts from (2) to (3). Fix the following code so it uses the appropriate load function:

import json

with open("dog.json") as f:
    dog = json.loads(f) # fixme

Check that type(dog) is a dict.

Now fix this one too:

data = '{"name": "Fido", "age": 1}'
dog = json.load(data) # fixme

And one more:

with open("dog.json") as f:
    data = f.read()
    dog = json.load(data) # fixme