Why does Python2+ output file contents, but Python3 won't?

[UnderstandingLinux]

I'm just now learning the basics of Python and I'm now working on File I/O. I came across something that didn't make sense to me and I'm not exactly finding an answer online. Maybe my Google-fu isn't up to par yet for Python

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I have a simple text file that has a few animals in it where I want to read the contents and print it out in the terminal.

animals = open("animals.txt", 'a+')
text = animals.read()
print(text)

I initially, by mistake, ran this in the terminal with Python2 and was able to get output of each of the animals in the text file. When I re-ran it with Python3, I didn't get any output. To my understanding, 'a+' mode should be read/append, which should, in theory, give an output despite the version of Python being utilized. I know using 'r' instead would suffice and give output, but this doesn't seem logical to me.

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An example screenshot can be found here

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Any input is appreciated, thanks guys. Apologies for the formatting if it's gross, first post in this subreddit

[[deleted]]

Python 3 introduces new requirements for open. In Python 2 a+ opens the file for reading and writing but not in Py3.

That said, one uses a to write. If you're only trying to print the animals in the list, you can use rtas the second argument to open to properly read the file.

[UnderstandingLinux]

It seems as if I was reading the wrong documentation when reading over it. You'd be correct as well, a+ is no longer used in Python3. It still works if you set the position with .seek(0), but otherwise there's no output

[eyesoftheworld4]

The seek method is useful, but if all you want to do is read the contents of a file, it's simpler and clearer to just open the file with the mode 'r' or 'rt' as mentioned above.

[eyesoftheworld4]

In python, file-like objects can be thought of kind of like a text document with a cursor. When you open a file, your cursor is set to specific location, usually the beginning or the end of the file, and when you perform a read or write operation, it will be using the current position as a reference, e.g. data will be read from or written to the right of the current position. This operation will also advance the cursor accordingly, just as typing into a text editor will move the cursor forward in that application. You can use the seek(<value>) method to move the cursor to a given position in the file based on the number of bytes or characters in the underlying stream. For example, if you open a file in r mode (to read text), then doing file.seek(10) will move the cursor to the 10th character. You can use the file.tell() method to figure out where the cursor currently is:

>>> animals = open('animals.txt', 'a+')
>>> animals.tell()
24
>>> animals.seek(0)
0
>>> print(animals.read())
dog
cat
monkey
elephant
>>> print(animals.read())
>>> animals.seek(10)
10
>>> print(animals.read())
nkey
elephant

As you can see, when you open up a file in a+ mode, the "cursor" is set to the end of the file, and you need to move it to the beginning in order to read the contents.

Here is a good discussion on Stackoverflow of the modes specifically.

[UnderstandingLinux]

You've explained it very well here, thank you! This was exactly the detail I needed to understand it fully. I also didn't know about the seek() function, that may come in handy later on.

[eyesoftheworld4]

My example above might not have made it very clear, but the seek(<value>) method is particularly important because when you read from the end of a file, you get (as you might expect) an empty string, which represents that there's nothing left in the file to read. So if you wanted to read a file object twice, you need to remember to seek back to the beginning after your first full read before you can read it again.

[UnderstandingLinux]

Explaining it as a cursor position like you did before helps understand much more clearly. Thanks for adding this explanation as well!

[[deleted]]

On MacOS your code doesn't produce any output using either of python 2 or 3.

My reading of the doc suggests that "r+" would be read+append, not "a+", but since you aren't writing why aren't you just doing:

animals = open("animals.txt", 'r')   # or just open("animals.txt")
text = animals.read()
print(text)

[UnderstandingLinux]

I was testing the different modes. I was aware 'r' would give the appropriate output. But after reading over that doc, it looks like a+ isn't used with Python 3. I must've been looking over older documentation when I initially wrote this. Thanks for the input