Reading and writing files
INPUT · Slides
01 / 07
read gives you the contents as one single string. Fine for a short note, but awkward the moment you want to do something per line.
"Add up every line with a score on it." "Show only the lines of three characters or more." What you want there is a way to take one line at a time.
02 / 07
The way to do it is almost disappointingly simple. Just write for line in f:. Exactly the same shape as looping over a list.
One thing to watch, though. Each line comes to you with its newline still attached. So print gives you blank lines in between.
with open("memo.txt", "w") as f: f.write("morning\nnoon\nnight\n")with open("memo.txt") as f: for line in f: print(line)Result
morning noon night
03 / 07
Take the strip from the last lesson and apply it to each line. Write line.strip() and that is it.
When you loop over a file with for, you write the strip alongside it. The pairing is common enough that you may as well learn it that way.
with open("memo.txt", "w") as f: f.write("morning\nnoon\nnight\n")with open("memo.txt") as f: for line in f: print(line.strip())Result
morning noon night
04 / 07
Put the code you have just written next to the code that loops over a list. Only what follows in is different — nothing else has moved.
That is not an accident. Lists, strings, dictionaries, files: Python is designed so the same for loops over all of them.
Things you can loop over share a common agreement, and for runs on that agreement alone. Chapter 8 gives away what it is. For now it is enough to notice that the same writing keeps working everywhere.
fruits = ["apple", "orange"]for fruit in fruits: print(fruit)with open("memo.txt", "w") as f: f.write("apple\norange\n")with open("memo.txt") as f: for line in f: print(line.strip())Result
apple orange apple orange
05 / 07
The other way to read is readlines, which hands back every line as a list.
Being a list, you can count it with len or take the first line with [0]. The newlines are still there, though, so you want a strip before you show anything.
with open("memo.txt", "w") as f: f.write("morning\nnoon\nnight\n")with open("memo.txt") as f: lines = f.readlines()print(lines)print(len(lines))Result
['morning\n', 'noon\n', 'night\n'] 3
06 / 07
Pick between the two like this.
for line in f: … read a line, deal with it, read the next. Light on however large a filef.readlines() … load every line into a list at once. For counting the lines, or looking back at them repeatedlyWhen in doubt, use for. Read a file of several million lines with readlines and every one of them lands in memory.
with open("memo.txt", "w") as f: f.write("morning\nnoon\nnight\n")with open("memo.txt") as f: lines = f.readlines()print(f"{len(lines)} lines")Result
3 lines
07 / 07
There were two ways to read a line at a time.
for line in f: and drop the newline with line.strip()f.readlines()And one more thing worth noticing. The for you learned in Chapter 2 worked on a file unchanged. Hold on to that feeling of "the same writing keeps working".
That covers reading. In the next group we take writing seriously: the difference between "w" and "a", and how people accidentally empty a file that mattered.