ይህ ትምህርት ገና ወደ አማርኛ አልተተረጎመም፤ ስለዚህ በእንግሊዝኛ ቀርቧል። የእንግሊዝኛውን ገጽ ክፈቱ

String methods

4 min read

Names typed into a form arrive messy: " hana ", "ABEL", a stray space at the end. By the end of this lesson you'll clean text, split it into words, join words back together, and search inside it, using the one rule that makes strings different from lists.

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Strings don't change

repr() shows a string the way you'd type it in code, with its quotes, so you can see every space. Predict what this prints.

ውጤቱን ገምቱ

Decide before you look. Guessing wrong is how this sticks.

name = "  Hana  "
name.strip()
print(repr(name))

Python prints

'  Hana  '

name.strip() makes a new string without the spaces, but nothing gives it a name, so it's lost. name still refers to the original string.

If you said 'Hana', you used the rule from lesson 4.2: prices.sort() changes the list. That rule is right for lists and wrong for strings.

strip() is a methodmethodA function that belongs to a value, called with a dot after it: name.strip(), prices.sort()., like the list methods in lesson 4.2. But a string method never changes the string it's called on. strip() does its work and returns a new string, the way vat(250) returned a number in lesson 5.1. To keep the result, give it a name:

name = "  Hana  "
name = name.strip()
print(repr(name))

Ctrl or Cmd + Enter runs

Read line 2 as "name becomes what name.strip() returns". Step through the two versions together: each arrow shows which value a name refers to.

Names and objects

Press Next to run one line at a time. Each arrow shows the object a name refers to.Each row is one step: the line that just ran, and the object each name refers to after it።

1name = "  Hana  "
2name.strip()
3name = name.strip()
4print(repr(name))

Step 1 of 4

Every step: the line that ran, and the object each name refers to after it
Step	Line	Names and objects
1	1	name → S1 ' Hana '
2	2	name → S1 ' Hana '
3	3	name → S2 'Hana' S1 ' Hana ' (no name refers to it now)
4	4	name → S2 'Hana'

On line 2 the new string is made and dropped at once, so the diagram never shows it. On line 3, name moves to a new string, and no name refers to the old one anymore.

The two rules are mirror images:

	`sort`, `append` (lesson 4.2)	`strip`, `upper`, `replace`
Changes the value it's called on?	Yes	Never
Returns	`None`	A new string
Right	`prices.sort()`	`name = name.strip()`
Wrong	`prices = prices.sort()`: `prices` becomes `None`	`name.strip()` alone: nothing changes

The reason is lesson 4.8: strings are immutableimmutableCan't be changed after it's made. Strings, numbers, and tuples are immutable; lists, dicts, and sets aren't.. No method can change a string, so a method that makes different text has to hand you a new string. (Methods that answer a question return other things: later in this lesson, find returns a number and split a list.)

ጥያቄ

After s = "tea" and then s.upper(), what is s?

Changing case

lower(), upper(), and title() each return a new string with the letters changed. title() makes the first letter of every word a capital and the rest small.

city = "adDIS abeba"
print(city.lower())
print(city.upper())
print(city.title())
print("ሰላም".upper())

Ctrl or Cmd + Enter runs

The last line is unchanged: Ge'ez has no capital and small letters, so there's nothing for upper() to change. Case belongs to some scripts, like Latin, not all. Change city to your own city, typed in mixed case, and run it again.

Because each method returns a string, you can call the next method on the result: " hana ".strip().title() strips first, then capitalizes, and gives 'Hana'.

Splitting and joining

split() cuts a string into a list of words. Look closely at the two spaces between the names, then predict both lines.

ውጤቱን ገምቱ

Decide before you look. Guessing wrong is how this sticks.

text = "Hana  Abel"
print(text.split())
print(text.split(" "))

Python prints

['Hana', 'Abel']
['Hana', '', 'Abel']

With no argument, split() cuts at every run of spaces, however long. split(" ") cuts at each single space, so two spaces in a row leave an empty string between them.

The text you cut at is the separatorseparatorThe text that goes between the pieces: what split cuts at, and what join puts back.. split(",") uses a comma, split(" ") exactly one space. To split words typed by a person, use split() with nothing inside: it also ignores spaces at the ends.

join() does the opposite: it glues a list of strings into one string. It reads backwards at first, because you call it on the separator:

names = ["Hana", "Abel", "Selam"]
print(", ".join(names))
print(" - ".join(names))

Ctrl or Cmd + Enter runs

join only takes strings. Try it on prices:

የተለመደ ስህተት

This breaks

prices = [250, 40, 1200]
print(" + ".join(prices))

Python says

TypeError: sequence item 0: expected str instance, int found

The fix

prices = [250, 40, 1200]
print(" + ".join([str(p) for p in prices]))

The sequence is the list you passed. Its item 0, 250, is an int, and join expected a str; it won't convert for you. A comprehension (lesson 4.7) makes a list of strings first.

Searching inside a string

find() returns the index where some text starts, or -1 if it isn't there at all. count() counts how many times it appears; like every search here, it's case-sensitive, so the E in ETB isn't counted. replace() returns a new string with every match replaced, not just the first:

sms = "You paid ETB 250 to Selam Coffee"
print(sms.find("ETB"))
print(sms.find("USD"))
print(sms.count("e"))
print("0911-23-45-67".replace("-", ""))

Ctrl or Cmd + Enter runs

Strings also have index(), which gives the same answer as find() when the text is there. When it isn't:

የተለመደ ስህተት

This breaks

sms = "You paid ETB 250 to Selam Coffee"
print(sms.index("USD"))

Python says

ValueError: substring not found

The fix

sms = "You paid ETB 250 to Selam Coffee"
print(sms.find("USD"))

A substring is a piece of a string. index raises when it isn't there; find returns -1, which you can check with an if.

startswith() checks the beginning. Give it a tuple and it checks each one. Ethiopian mobile numbers start with 09 or 07:

print("0712345678".startswith(("09", "07")))
print("0112345678".startswith(("09", "07")))

Ctrl or Cmd + Enter runs

መልመጃ

Clean the sign-up list

People typed their names into a sign-up form. Strip the spaces, fix the case, and print all the names on one line, separated by a comma and a space.

Output

Hana, Abel, Selam Tesfaye

Your code

typed = ["  hana ", "ABEL", " selam tesfaye "]
# Clean each name, then join them

Ctrl or Cmd + Enter runs

መፍትሄውን አሳይ

This is one way to solve it, not the only one. If yours prints the same thing, it works.

typed = ["  hana ", "ABEL", " selam tesfaye "]
clean = []
for name in typed:
    clean.append(name.strip().title())
print(", ".join(clean))

ዋና ዋና ነጥቦች

A method is a function called with a dot: name.strip().
Strings are immutable, so string methods never change the string; they return a new one. Keep it with name = name.strip().
split() cuts at any run of spaces; split(" ") cuts at every single space. ", ".join(words) is called on the separator and takes only strings.
find returns -1 when the text is missing; index raises ValueError. replace replaces every match.
Ge'ez letters have no case, so upper() and lower() leave them unchanged.