Note that this line of code can leave the file open for reading. Try using a with statement.

f = open('myfile.txt')

Reference: What is the python “with” statement designed for?

The other answers are not wrong. I wanted to add a solution with the recently introduced assignment expression "walrus operator" in Python 3.8.

Reference: Assignment Expressions: The Walrus Operator

def read_file():
    chunk_size = int(input("How many letters do you want to read each time?: "))
    with open('myfile.txt') as fh:
        while (contents := fh.read(chunk_size)):
            print(contents)
read_file()

Output:

to be or
 not to 
be that 
is the q
uestion

Sum up the length of all words in a line:

characters += sum(len(word) for word in wordslist)

The whole program:

with open('my_words.txt') as infile:
    lines=0
    words=0
    characters=0
    for line in infile:
        wordslist=line.split()
        lines=lines+1
        words=words+len(wordslist)
        characters += sum(len(word) for word in wordslist)
print(lines)
print(words)
print(characters)

Output:

3
13
35

This:

(len(word) for word in wordslist)

is a generator expression. It is essentially a loop in one line that produces the length of each word. We feed these lengths directly to sum:

sum(len(word) for word in wordslist)

Improved version

This version takes advantage of enumerate, so you save two lines of code, while keeping the readability:

with open('my_words.txt') as infile:
    words = 0
    characters = 0
    for lineno, line in enumerate(infile, 1):
        wordslist = line.split()
        words += len(wordslist)
        characters += sum(len(word) for word in wordslist)

print(lineno)
print(words)
print(characters)

This line:

with open('my_words.txt') as infile:

opens the file with the promise to close it as soon as you leave indentation. It is always good practice to close file after your are done using it.

with open(filename) as f:
    while True:
        c = f.read(1)
        if not c:
            print("End of file")
            break
        print("Read a character:", c)

The output seems weird because it is printing the new lines too. If you count the number of characters it prints, the output is correct when considering the newline characters.

When you call print(), python will add a newline to the output. Below is what python is seeing every time you call print:

>>> f.read(2):  'He'+\n              <- 2 characters + newline
>>> f.read(3):  'llo'+\n             <- 3 characters + newline
>>> f.read(8):  ' World\nT'+\n       <- 8 characters + newline
>>> f.read(15): 'his is our new '+\n <- 15 characters + newline
>>> 
>>> f.read():   'text file\nand this is another line.\nWhy? Because we can.'+\n

When you call read on a file, it moves the cursor position by the specified number of characters. When you call read again, it will pick back up where it left off. When you don't specify a number, it will read until the end of the file.