Reading & Writing Files

Everything your programs have done so far vanishes the moment they finish - the variables are forgotten, the results gone. To do anything useful with data, a program needs to read information from disk and write its results back. Saving a watchlist, loading yesterday's prices, exporting a report: all of it is file handling, and Python makes it refreshingly simple. This is the bridge between a throwaway script and a real tool.

Writing a file

To write to a file you open it in "w" (write) mode and call write. The modern way always wraps this in a with block, for a reason we'll see in a moment:

# Writing a file: open it in "w" (write) mode, inside a with-block.
lines = ["RELIANCE,1313.60", "TCS,2109.00", "INFY,1056.60"]

with open("watchlist.txt", "w") as f:     # "w" creates the file (or overwrites it)
    for line in lines:
        f.write(line + "\n")              # write() does NOT add newlines for you

print("Wrote", len(lines), "lines to watchlist.txt")

Wrote 3 lines to watchlist.txt

open("watchlist.txt", "w") creates the file (in the current folder), and f.write(...) puts text into it. Two things to note: write does not add line breaks for you, so we tack on "\n" ourselves; and "w" mode starts the file fresh.

The mode letter you pass to open decides what happens:

The with block, and why it matters

You'll notice every example uses with open(...) as f:. The with block automatically closes the file when you're done - even if an error happens partway through. An open file that's never closed can lose data that's still sitting in a buffer, or lock the file from other programs.

Reading a file back

Reading is the mirror image - open in "r" mode (or omit the mode, since read is the default). You can grab the whole file at once with read(), or, more usefully for data, loop over it line by line:

# Reading a file back: open it in "r" (read) mode - the default.
with open("watchlist.txt", "r") as f:
    content = f.read()                # the whole file as one big string
print("--- whole file ---")
print(content)

# More often you loop line by line, parsing as you go.
print("--- parsed ---")
with open("watchlist.txt") as f:      # "r" is assumed if you omit the mode
    for line in f:
        symbol, price = line.strip().split(",")
        print(f"{symbol:10} {float(price):>9.2f}")

--- whole file ---
RELIANCE,1313.60
TCS,2109.00
INFY,1056.60

--- parsed ---
RELIANCE     1313.60
TCS          2109.00
INFY         1056.60

Looping over the file object hands you one line at a time, including its trailing newline - which is why we strip() it before split(","). That two-step (strip then split) is the bread-and-butter of reading simple data files by hand.

Appending

To add to a file without destroying what's there, open it in "a" (append) mode. New writes land at the end:

# "a" (append) mode adds to the end without erasing what is already there.
with open("watchlist.txt", "a") as f:
    f.write("HDFCBANK,1642.40\n")

# Read it back to confirm the new line joined the others.
with open("watchlist.txt") as f:
    print(f.read())

RELIANCE,1313.60
TCS,2109.00
INFY,1056.60
HDFCBANK,1642.40

HDFCBANK joins the three names already in the file, instead of replacing them. Append mode is perfect for logs - a running record you keep adding lines to, like a journal of trades or signals.

A note on file paths

"watchlist.txt" is a relative path - it points to a file in the folder your program is running from. You can also give an absolute path like "C:/data/watchlist.txt" (forward slashes work fine on Windows in Python, and avoid the backslash headaches). For anything beyond the basics, Python's pathlib module - with its handy Path object - is the modern, tidy way to build and join paths; keep the name in mind for when your projects grow.

Try it yourself

• Write your three favourite stock symbols to a file mylist.txt, one per line, then read it back and print each in uppercase.
• Open the same file in "a" mode and add a fourth symbol. Read it again to confirm all four are there.
• Now open it in "w" mode and write a single symbol. Read it back - notice the other three are gone. (That's the overwrite warning, made real.)

Recap

• Open files with with open(name, mode) as f: - the with block always closes them safely.
• Modes: "r" read, "w" write (which overwrites), "a" append (adds to the end).
• Write with f.write(text) (add your own "\n"); read with f.read() or by looping line by line (then strip() and split()).
• Paths can be relative (to the current folder) or absolute; use forward slashes, and pathlib for bigger projects.

You just hand-parsed comma-separated text with split(","). There's a more structured cousin of that format used everywhere in trading - JSON, the language market APIs speak. We've glimpsed it twice already; in the next chapter we give it the full treatment.