Unexpected Python Output in File Handling

Question

I have the following function in python. It's a simple function that reads a file. I coded it to act differently when the file is empty. When the file is non-empty, len(f.read()) has a non zero value but still it gets into the if condition.

Code:

def data():
    with open('data.dat','r') as f:
        print (len(f.read()))

        print(len(f.read())==0)

        if len(f.read())==0:
            print ("IF Entered")
        else:
            print ("Else Entered")

    return {}

Output:

19
True
IF Entered

Expected Output:

19
False
Else Entered

EDIT: I updated my code(I wanted to use the f twice for json.load but when I used it once, it went to the end to give crazy outputs. Now this gets the work done.) to:

def data():
    with open('data.dat','r') as f:
        if len(f.read())==0:
            return {'post':{}}
    with open('data.dat','r') as f:
        return json.load(f)

Possible duplicate of [Why can't I call read() twice on an open file?](https://stackoverflow.com/questions/3906137/why-cant-i-call-read-twice-on-an-open-file) — Georgy, May 20 '19 at 08:00

vishes_shell · Accepted Answer · 2019-05-20T08:02:23.180

3

This happens because when you call f.read() this will move your cursor to the end of the file, and in the second run nothing would be read.

This example is also presented in the docs:

To read a file’s contents, call f.read(size), which reads some quantity of data and returns it as a string (in text mode) or bytes object (in binary mode). size is an optional numeric argument. When the size is omitted or negative, the entire contents of the file will be read and returned; it’s your problem if file is twice as large as your machine’s memory. Otherwise, at most size bytes are read and returned. If the end of the file has been reached, f.read() will return an empty string ('').

So as OP correctly asked, you need to assign the value of .read() to any variable and process it as you wish.

def data():
    with open('data.dat','r') as f:
        contents = f.read()

    # do anything with your `contents`

    return {}

Update to OP's EDIT (with solution) :

You can omit second time read just by catching decode error (if file is empty):

def data():
    with open('data.dat', 'r') as f:
        try:
            return json.load(f)
        except json.JSONDecodeError:
            return {'post': {}}

edited May 20 '19 at 08:02

answered May 20 '19 at 07:39

vishes_shell

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So I should assign it's content to a variable right? – Abhinay Pandey May 20 '19 at 07:40
@AbhinayPandey you are completely right! I've update my answer. – vishes_shell May 20 '19 at 07:41
What if I need to use the `f` in `json.load(f)`, then if I'll once assign it to the content the pointer will go to the end, then json.load will throw an error?! P.S. Should I ask this in another question? – Abhinay Pandey May 20 '19 at 07:47
nvm I got the solution. Check my EDIT in the question – Abhinay Pandey May 20 '19 at 07:54
@AbhinayPandey i suggest you a better solution, that don't require reading file twice. – vishes_shell May 20 '19 at 08:02

Unexpected Python Output in File Handling

1 Answers1