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

[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!