String methods and f-strings

Lesson 10 of 62, about 3 min

Strings come with dozens of methods. A method is called with a dot after the value: text.upper(). Since strings are immutable, every method returns a new string and leaves the original alone.

Changing case

python

s = "machine Learning"
print(s.upper())
print(s.lower())
print(s.title())
print(s.capitalize())
print(s.swapcase())

Trimming whitespace

python

raw = "   padded   "
print(repr(raw.strip()))
print(repr(raw.lstrip()))
print(repr(raw.rstrip()))

strip() also accepts characters to remove: "xxhixx".strip("x") gives "hi".

Searching

python

s = "data science is data driven"
print(s.find("data"))       # index of first match, -1 if missing
print(s.rfind("data"))      # last match
print(s.count("data"))
print(s.startswith("data"))
print(s.endswith("driven"))
print(s.index("science"))   # like find, but raises ValueError if missing

Replacing

python

s = "I like Java"
print(s.replace("Java", "Python"))
print("aaa".replace("a", "b", 2))   # replace at most 2 occurrences

Splitting and joining

split() turns a string into a list; join() does the reverse.

python

csv = "apple,banana,cherry"
parts = csv.split(",")
print(parts)

words = "one two  three".split()   # no argument splits on any whitespace
print(words)

print(" | ".join(parts))
print("".join(["a", "b", "c"]))

Checking content

python

print("12345".isdigit())
print("hello".isalpha())
print("hello123".isalnum())
print("   ".isspace())
print("HELLO".isupper())

f-strings

An f-string is a string with an f before the opening quote. Anything inside { } is evaluated as Python.

python

name = "Sara"
marks = 87.456
print(f"{name} scored {marks} marks")
print(f"{name} scored {marks:.1f} marks")     # one decimal place
print(f"{name!r}")                             # repr of the value
print(f"{2 ** 10}")                            # any expression works

Format specs

The part after the colon controls how the value looks.

python

n = 1234567.891
print(f"{n:,.2f}")       # thousands separator, 2 decimals
print(f"{0.256:.1%}")    # percentage
print(f"{42:05d}")       # zero padded to width 5
print(f"{'left':<10}|")  # left aligned in 10 characters
print(f"{'right':>10}|")
print(f"{'mid':^10}|")

Debugging shortcut

{expr=} prints the expression and its value together:

python

x = 10
y = 3
print(f"{x=}, {y=}, {x * y=}")

The older format() method

You will still see this in existing code. It works the same way but is more verbose.

python

print("{} scored {:.1f}".format("Sara", 87.456))
print("{name} is {age}".format(name="Amit", age=20))

Practice

Take the string " Machine Learning With Python ", strip it, lowercase it and replace spaces with hyphens to make a URL slug.
Given price = 1499.5, print it as Rs 1,499.50 using an f-string.
Count how many times the letter "a" appears in "banana bandana".