Strings and f-strings

Text is a string

A string (str) is text. Write it between single or double quotes: both mean the same, so pick one style and keep it. A backslash starts an escape, such as \n for a new line. len() counts the characters. Strings are immutable: you never change one in place, you build a new one.

symbol = "AAPL"
note = 'bid and ask'
print(symbol, len(symbol))  # AAPL 4
print(note)                 # bid and ask

Indexing and slicing

Each character has a position, counting from 0. text[0] is the first character and text[-1] the last. A slice text[start:stop] takes the characters from start up to, but not including, stop. Leave one side empty to run to that end.

# Indexing starts at 0; a slice stops before its end index
ticker = "AAPL.US"
print(ticker[0], ticker[-1])   # A S
print(ticker[:4], ticker[5:])  # AAPL US

Methods that clean and split text

Methods are functions attached to a value and called with a dot. They return a new string and leave the original alone. strip() removes spaces and newlines around the text, upper() and lower() change the case, replace(old, new) swaps text, and split(",") cuts a line at every comma into a list of pieces. Data feeds are full of lines like this.

# Clean up a raw line from a data feed
raw = "  aapl,184.25,200 \n"
parts = raw.strip().split(",")
print(parts)             # ['aapl', '184.25', '200']
print(parts[0].upper())  # AAPL
print(float(parts[1]))   # 184.25

# join is the opposite of split; in tests for a piece of text
print("-".join(["AAPL", "MSFT"]))  # AAPL-MSFT
print("AAPL" in "AAPL,MSFT")       # True

f-strings put values into text

Put an f before the opening quote and write any expression between { and }: Python evaluates it and inserts the result as text. This replaces gluing pieces together with + and converting numbers by hand. Anything that is a valid expression works inside the braces, even arithmetic.

# f-strings put values straight into text
symbol, bid, ask = "AAPL", 184.25, 184.5
print(f"{symbol} {bid} x {ask}")       # AAPL 184.25 x 184.5
print(f"{symbol} spread {ask - bid}")  # AAPL spread 0.25

Format specifications

After a colon inside the braces, a format specification controls how the value looks. .2f shows two decimals, , adds thousands separators, + always shows the sign, >8 right-aligns in a width of 8, and .1% shows a fraction as a percentage. They combine: +,.2f is a signed amount with separators and two decimals.

# After the colon, a format spec controls the look
price, pnl, quantity = 184.5, 1234.5, 200
print(f"{price:.2f}")       # 184.50
print(f"{pnl:+,.2f}")       # +1,234.50
print(f"{quantity:>8d}|")   # "     200|"
print(f"{0.0123:.1%}")      # 1.2%

Next up: booleans and comparisons, the True and False values that every decision in a trading rule is built from.

Lab · what's graded

• Write a quote as text with quote_line25 pts
• Show profit and loss with a sign using signed_pnl25 pts
• Clean a ticker symbol with normalize_symbol25 pts
• Read a fill line from a feed with parse_fill25 pts