What is the difference between opening and reading a file in python?

"Opening" a file doesn't actually bring any of the data from the file into your program. It just prepares the file for reading (or writing), so when your program is ready to read the contents of the file it can do so right away.

Opening a file allows you to read or write to it (depending on the flag you pass as the second argument), whereas reading it actually pulls the data from a file that is typcially saved into a variable for processing or printed as output.

You do not always read from a file once it is opened. Opening also allows you to write to a file, either by overwriting all the contents or appending to the contents.

To read from a file:

>>> myfile = open('foo.txt', 'r')
>>> myfile.read()

First you open the file with read permission (r) Then you read() from the file

To write to a file:

>>> myfile = open('foo.txt', 'r')
>>> myfile.write('I am writing to foo.txt')

The only thing that is being done in line 1 of each of these examples is opening the file. It is not until we actually read() from the file that anything is changed

open gets you a fd (file descriptor), you can read from that fd later.

One may also open a file for other purpose, say write to a file.

It seems to me you can read lines from the file handle without invoking the read method but I guess read() truly puts the data in the variable location. In my course we seem to be printing lines, counting lines, and adding numbers from lines without using read().

The rstrip() method needs to be used, however, because printing the line from the file handle using a for in statement also prints the invisible line break symbol at the end of the line, as does the print statement.

From Python for Everybody by Charles Severance, this is the starter code.

  """
7.2
Write a program that prompts for a file name,
then opens that file and reads through the file,
looking for lines of the form:
X-DSPAM-Confidence:    0.8475
Count these lines and extract the floating point
values from each of the lines and compute the
average of those values and produce an output as
shown below. Do not use the sum() function or a
variable named sum in your solution.
You can download the sample data at
http://www.py4e.com/code3/mbox-short.txt when you
are testing below enter mbox-short.txt as the file name.
"""


# Use the file name mbox-short.txt as the file name

fname = input("Enter file name: ") 
fh = open(fname) 
for line in fh:
    if not line.startswith("X-DSPAM-Confidence:") : 
        continue
    print(line)
print("Done")