Text: Strings in Depth ✂️

f-strings: the only way you need

Putting values inside text used to be awkward. Since Python 3.6 there is one obviously correct way, and it is lovely. Put an f before the quote and then put expressions inside curly braces.

name = "Guybrush"
insults = 7

print(f"{name} knows {insults} insults.")
print(f"After tonight, {insults + 3}.")
print(f"His name in capitals is {name.upper()}.")

Guybrush knows 7 insults.
After tonight, 10.
His name in capitals is GUYBRUSH.

Anything that produces a value can go in the braces: maths, function calls, anything.

Formatting inside the braces

price = 1234.5678
ratio = 0.8734
name = "grog"

print(f"{price:.2f}")        # two decimal places
print(f"{price:,.2f}")       # thousands separators too
print(f"{ratio:.1%}")        # as a percentage
print(f"{name:>12}|")        # right aligned in 12 characters
print(f"{name:<12}|")        # left aligned
print(f"{name:^12}|")        # centred
print(f"{42:04d}")           # padded with zeros

1234.57
1,234.57
87.3%
        grog|
grog        |
    grog    |
0042

Those alignment tools are how you print a table that lines up without fighting it. There is a whole format specification mini-language behind them; the seven above cover nearly everything you will ever need.

total = 47
items = 3
print(f"{total=} {items=} {total / items=:.2f}")

total=47 items=3 total / items=15.67

Strings are sequences

A string is a row of characters, each with a numbered position. Python counts from zero, which feels wrong for about a week and then feels obvious forever.

word = "MONKEY"
#        012345
#       -654321   (negative numbers count from the right)

print(word[0])
print(word[3])
print(word[-1])
print(len(word))

M
K
Y
6

Slicing: taking a piece

title = "The Secret of Monkey Island"

print(title[0:3])       # from 0, up to but NOT including 3
print(title[4:10])      # characters 4 through 9
print(title[:3])        # from the start
print(title[14:])       # to the end
print(title[-6:])       # the last six
print(title[::2])       # every second character
print(title[::-1])      # backwards

The
Secret
The
Monkey Island
Island
TeSce fMne sad
dnalsI yeknoM fo terceS ehT

The rule that trips everyone: the start is included, the end is not. [0:3] gives you three characters. It looks arbitrary until you notice [0:3] and [3:6] fit together perfectly with no gap and no overlap, which is why it was chosen.

Strings cannot be changed

word = "grog"
# word[0] = "f"     # this raises TypeError

better = "f" + word[1:]
print(word)
print(better)

grog
frog

Strings are immutable: every operation makes a new string rather than editing the old one. This sounds like a limitation and is actually a gift. It means a string handed to a function cannot be secretly altered under your feet, which removes a whole species of bug.

The methods that do the real work

messy = "   The Governor, ELAINE Marley   "

print(messy.strip())
print(messy.strip().lower())
print(messy.strip().upper())
print(messy.strip().title())
print(messy.strip().replace("Marley", "Threepwood"))
print(len(messy), len(messy.strip()))

The Governor, ELAINE Marley
the governor, elaine marley
THE GOVERNOR, ELAINE MARLEY
The Governor, Elaine Marley
The Governor, ELAINE Threepwood
33 27

split and join, the two workhorses

crew = "Guybrush,Elaine,Otis,Meathook"

members = crew.split(",")
print(members)
print(len(members), "crew members")

print(" and ".join(members))
print("\n".join(members))

['Guybrush', 'Elaine', 'Otis', 'Meathook']
4 crew members
Guybrush and Elaine and Otis and Meathook
Guybrush
Elaine
Otis
Meathook

split and join are opposites, and between them they handle a startling proportion of real-world data work. Note the slightly backwards-looking separator.join(list) order. Everyone writes it the wrong way round the first ten times.

Escapes: characters with special jobs

print("Line one\nLine two")
print("Column\tone\tColumn two")
print("She said \"hello\" to me.")
print('It\'s fine with single quotes too.')
print("A backslash: \\")
print(r"A raw string ignores escapes: C:\new\table")

Line one
Line two
Column	one	Column two
She said "hello" to me.
It's fine with single quotes too.
A backslash: \
A raw string ignores escapes: C:\new\table

That last one matters on Windows and in Lesson 25 on regular expressions. "C: ew" contains a newline, because \n is special. r"C: ew" is the literal text you meant.

Multi-line strings

sign = """
   ==========================
     STAN'S PREVIOUSLY OWNED
        VESSELS
   ==========================
"""
print(sign)

   ==========================
     STAN'S PREVIOUSLY OWNED
        VESSELS
   ==========================

Three quotes let a string span lines. These are also how Python documents functions (Lesson 17) and how you will write prompts for a language model in Level 6.

raw = "  ELAINE.MARLEY@melee.gov  "

email = raw.strip().lower()
user, domain = email.split("@")

print(f"clean:  {email}")
print(f"user:   {user}")
print(f"domain: {domain}")

clean:  elaine.marley@melee.gov
user:   elaine.marley
domain: melee.gov

full = "guybrush ulysses threepwood"

parts = full.split()
initials = ".".join(part[0].upper() for part in parts) + "."
print(initials)

G.U.T.

phrase = "Never odd or even"

cleaned = phrase.lower().replace(" ", "")
print(cleaned)
print(cleaned == cleaned[::-1])

neveroddoreven
True