2.2 Files

Big ideas

• with open("data.txt") as f: opens a file and closes it automatically at the end of the block. Modes: "r" read (default), "w" write (erases!), "a" append.
• Reading: for line in f: visits each line (with its trailing \n, so strip() it); f.read() gives the whole text; f.readlines() a list of lines.
• Writing: f.write(text) writes exactly what you give it, so add \n yourself. Numbers must be converted with str() or an f-string.
• CSV-style data: split each line on the delimiter, line.strip().split(","), convert the fields you need, and accumulate totals, counts or a list of records.
• A missing file raises FileNotFoundError; check the name and that the file sits next to your program (or give a full path).

Code you should know

with open("scores.txt", "w") as f:
    f.write("Ana,91\n")
    f.write("Ben,78\n")
    f.write("Cy,85\n")

total = 0
count = 0
with open("scores.txt") as f:
    for line in f:
        name, score = line.strip().split(",")
        total += int(score)
        count += 1
        print(name, "->", score)
print("average", total / count)

Ana -> 91
Ben -> 78
Cy -> 85
average 84.66666666666667

with open("poem.txt", "w") as f:
    f.write("Roses are red\nviolets are blue\n")

with open("poem.txt") as f:
    lines = f.readlines()
print(len(lines), "lines")
words = 0
for line in lines:
    words += len(line.split())
print(words, "words")

with open("poem.txt", "a") as f:
    f.write("sugar is sweet\n")
print(open("poem.txt").read())

2 lines
6 words
Roses are red
violets are blue
sugar is sweet

Worked example

total = 0
with open("nums.txt") as f:
    for line in f:
        total += int(line)
print(total)

Common mistakes

• Opening with "w" when you meant to add: the old contents are gone.
• Forgetting strip(), so the last field keeps a newline and "85\n" == "85" is False.
• Writing a number with f.write(42) (TypeError); write a string.
• Reading the file twice without reopening it: the second loop sees nothing.