To read a file in Python, the file must be first opened, and then a read() function is needed. Why is that when we use a for loop to read lines of a file, no read() function is necessary?
filename = 'pi_digits.txt'
with open(filename,) as file_object:
for line in file_object:
print(line)
I'm used to the code below, showing the read requirement.
for line in file_object.read():
This is because the file_object class has an "iter" method built in that states how the file will interact with an iterative statement, like a for loop.
In other words, when you say for line in file_object the file object is referencing its __iter__ method, and returning a list where each index contains a line of the file.
Python file objects define special behavior when you iterate over them, in this case with the for loop. Every time you hit the top of the loop it implicitly calls readline(). That's all there is to it.
Note that the code you are "used to" will actually iterate character by character, not line by line! That's because you will be iterating over a string (the result of the read()), and when Python iterates over strings, it goes character by character.
The open command in your with statement handles the reading implicitly. It creates a generator that yields the file a record at a time (the read is hidden within the generator). For a text file, each line is one record.
Note that the read command in your second example reads the entire file into a string; this consumes more memory than the line-at-a-time example.
file_object.read() returns a single string, whose iterator returns a single character at a time, where as a file iterator actually returns a line at a time.
with has nothing to do with it. Open the file any other way and the loop will behave the same.with open ... as certainly opens the file (specifically, the call to open does; the with statement merely ensures it will be closed as well), although it is the resulting file object (or whatever open ends up returning in Python 3) which produces the iterator used by the for loop to actually read from the file.with plays a role.
readwithout an argument.