Searching & Checking — in, find, startswith, isdigit

By the end of this lesson you can:

• Test whether a string contains something using in.
• Find where something sits with find(), and handle the -1 it returns when it is missing.
• Check the shape of input with startswith(), endswith() and isdigit() before converting it.

Lesson 3 converted a field with int(). What happens when the field is not what you assumed?

age = int("13")
print(age + 1)

age = int("thirteen")
print(age + 1)

14
Traceback (most recent call last):
ValueError: invalid literal for int() with base 10: 'thirteen'

Like checking a form before you file it. Is the name box filled in? Does the phone number start with 01? Is the age box actually digits? Each is one question with a yes-or-no answer.

in — is it in there at all?

order = "pizza with extra hot sauce"

print("hot sauce" in order)
print("chicken" in order)
print("HOT SAUCE" in order)

Output

True
False
False

The third line matters: in is case sensitive. Lower both sides first — lesson 2 — if case should not count.

find() — whereabouts is it?

full_name = "Leo Reed"

print(full_name.find(" "))
print(full_name.find("z"))

Output

3
-1

Shape checks

phone = "0123456789"
code = "ADV002-0147"

print(phone.startswith("01"))
print(code.startswith("ADV"))
print(code.endswith("0147"))
print(phone.isdigit())
print(code.isdigit())
print("Mia".isalpha())
print("".isdigit())

Output

True
True
True
True
False
True
False

The family is larger than these — isalnum(), isspace(), islower() — and they all answer True or False. Note the last line: an empty string is not digits, which is exactly the answer you want when someone presses Enter without typing.

count() — how many?

sentence = "the rain in spain stays mainly in the plain"

print(sentence.count("ain"))
print(sentence.count("the"))

Output

4
2

Why it matters. Every one of these guards a line that would otherwise crash or quietly do the wrong thing. Checking first is cheaper than debugging later.

Four entries, each checked before it is trusted. Nothing here can crash.

# check_form.py - validate before you convert

entries = [
    ["Mia Tom", "0123456789", "13"],
    ["Leo", "60123456789", "14"],
    ["Ana", "01234", "thirteen"],
    ["", "0198887777", "12"],
]

for name, phone, age in entries:
    print(f"Checking {name!r}")

    if name.strip() == "":
        print("   Name is empty.")
    elif not name.replace(" ", "").isalpha():
        print("   Name has characters that are not letters.")
    else:
        print(f"   Name OK: {name.strip().title()}")

    if not phone.isdigit():
        print("   Phone must be digits only.")
    elif not phone.startswith("01"):
        print("   mobiles start with 01.")
    elif len(phone) < 10 or len(phone) > 11:
        print(f"   Phone should be 10 or 11 digits, got {len(phone)}.")
    else:
        print(f"   Phone OK: {phone}")

    if age.isdigit():
        print(f"   Age OK: {int(age)} - next year {int(age) + 1}")
    else:
        print(f"   Age {age!r} is not a number.")

    print()

Output

Checking 'Mia Tom'
   Name OK: Mia Tom
   Phone OK: 0123456789
   Age OK: 13 - next year 14

Checking 'Leo'
   Name OK: Leo
   Phone: mobiles start with 01.
   Age OK: 14 - next year 15

Checking 'Ana'
   Name OK: Ana
   Phone should be 10 or 11 digits, got 5.
   Age 'thirteen' is not a number.

Checking ''
   Name is empty.
   Phone OK: 0198887777
   Age OK: 12 - next year 13

Three habits worth copying:

• int(age) only runs inside the if. The check and the conversion are never separated, so the crash cannot happen.
• elif orders the checks from cheapest to most specific, and each entry gets exactly one phone message rather than three.
• {name!r} shows the quotes, so an empty name is visible as '' rather than nothing at all.

in asks whether text contains something; find() says where and returns -1 when it does not, which must always be tested for. startswith(), endswith() and the is...() family check the shape of input, so you can convert only what is safe to convert. All of them are case sensitive.