TopolearnMost scripts read input files, logs or CSVs and write output somewhere.
The built-in open(path, mode, encoding=...) returns a file object. Always use it in a with block, which closes the file automatically when the block ends, even if an exception occurs:
with open("notes.txt", encoding="utf-8") as f:
text = f.read()The mode says what you're doing:
"r"read (default),"w"write (truncates an existing file!),"a"append,"x"create but fail if it exists- add
"b"for binary data such as images:"rb","wb"— you then read and writebytes, notstr
Always pass encoding="utf-8" for text. Without it Python uses the platform's default, which on Windows may not be UTF-8, so the same script can behave differently on another machine.
To read, f.read() returns the whole content as one string; iterating for line in f: streams one line at a time and works for files of any size. Each line keeps its trailing \n, so call line.rstrip("\n") or line.strip(). f.readlines() loads all lines into a list.
To write, f.write(s) writes a string and does not add a newline for you; print(..., file=f) does.
For structured formats, use the standard library instead of hand-parsing: csv.reader/csv.DictReader (open with newline=""), and json.load/json.dump. Later you'll meet pathlib.Path, which offers shortcuts like Path("a.txt").read_text(encoding="utf-8").
Example
import json
with open("todo.txt", "w", encoding="utf-8") as f:
f.write("buy milk\n")
print("call mum", file=f) # print adds the newline
with open("todo.txt", encoding="utf-8") as f:
for number, line in enumerate(f, start=1):
print(number, line.rstrip("\n"))
# 1 buy milk
# 2 call mum
with open("todo.txt", "a", encoding="utf-8") as f:
f.write("water plants\n")
config = {"debug": True, "retries": 3}
with open("config.json", "w", encoding="utf-8") as f:
json.dump(config, f, indent=2)
with open("config.json", encoding="utf-8") as f:
print(json.load(f)["retries"]) # 3Remember
- Open files in a
withblock so they always close - Pass
encoding="utf-8"for text files - Iterate
for line in fto stream large files
Watch out for
- Opening an existing file with mode
"w"and wiping its contents by accident.