Common string transformations in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the str.upper() method do in Python?

It converts all lowercase letters in a string to uppercase letters. For example, 'hello'.upper() returns 'HELLO'.

beginner

How can you remove whitespace from the start and end of a string in Python?

Use the str.strip() method. It removes spaces, tabs, and newlines from both ends of the string. For example, ' hello '.strip() returns 'hello'.

intermediate

What is the difference between str.replace(old, new) and str.translate()?

str.replace(old, new) replaces all occurrences of a substring with another substring. str.translate() replaces characters based on a translation table, useful for multiple character replacements at once.

beginner

How do you split a string into a list of words in Python?

Use the str.split() method without arguments. It splits the string at whitespace and returns a list of words. For example, 'a b c'.split() returns ['a', 'b', 'c'].

beginner

What does str.join(iterable) do?

It joins elements of an iterable (like a list) into a single string, inserting the original string between elements. For example, '-'.join(['a', 'b', 'c']) returns 'a-b-c'.

Which method converts all characters in a string to lowercase?

A str.strip()

B str.lower()

C str.capitalize()

D str.upper()

The str.lower() method converts all letters to lowercase.

What does ' hello '.strip() return?

A 'hello'

B ' hello '

C 'hello '

D ' hello'

The strip() method removes spaces from both ends.

How do you split the string 'one,two,three' into a list by commas?

A 'one,two,three'.join(',')

B 'one,two,three'.split()

C 'one,two,three'.split(',')

D 'one,two,three'.replace(',', ' ')

Using split(',') splits the string at commas.

What does '-'.join(['a', 'b', 'c']) produce?

A 'a-b-c'

B 'abc'

C 'a b c'

D '-a-b-c-'

The join() method inserts the string between list elements.

Which method replaces all occurrences of 'cat' with 'dog' in a string?

A str.translate('cat', 'dog')

B str.split('cat')

C str.strip('cat')

D str.replace('cat', 'dog')

replace() swaps all 'cat' substrings with 'dog'.

Explain how to convert a string to uppercase and then remove spaces from both ends.

Think about calling one method after another on the string.

Describe how to split a sentence into words and then join them back with commas.

First break the string, then combine with a new separator.

Practice

1. Which Python string method converts all characters in a string to uppercase?
text = "hello world"

easy

A. text.upper()

B. text.lower()

C. text.strip()

D. text.replace()

Solution

1. Step 1: Understand the purpose of each method

   upper() converts all letters to uppercase, lower() to lowercase, strip() removes spaces, replace() changes parts of the string.

2. Step 2: Identify the method that changes all letters to uppercase

   upper() is the method that does this.

3. Final Answer:

   text.upper() -> Option A

4. Quick Check:

   upper() = text.upper() [OK]

Hint: Uppercase all letters with upper() method [OK]

Common Mistakes:

• Confusing upper() with lower()
• Using strip() to change case
• Trying replace() without arguments

2. Which of the following is the correct syntax to remove whitespace from both ends of the string text?

easy

A. text.strip()

B. text.trim()

C. text.remove()

D. text.cut()

Solution

1. Step 1: Recall Python string methods for trimming spaces

   Python uses strip() to remove whitespace from both ends of a string.

2. Step 2: Check the options for correct method name

   trim(), remove(), and cut() are not valid Python string methods.

3. Final Answer:

   text.strip() -> Option A

4. Quick Check:

   strip() = text.strip() [OK]

Hint: Use strip() to remove spaces from start and end [OK]

Common Mistakes:

• Using trim() which is not a Python method
• Trying remove() or cut() which don't exist
• Confusing strip() with replace()

3. What is the output of the following code?

text = "apple,banana,cherry"
result = text.split(",")
print(result)

medium

A. apple,banana,cherry

B. ['apple,banana,cherry']

C. apple banana cherry

D. ['apple', 'banana', 'cherry']

Solution

1. Step 1: Understand split() method with comma separator

   split(",") breaks the string at each comma, creating a list of parts.

2. Step 2: Apply split to the string

   Splitting "apple,banana,cherry" by comma gives ['apple', 'banana', 'cherry'].

3. Final Answer:

   ['apple', 'banana', 'cherry'] -> Option D

4. Quick Check:

   split(",") = list of words [OK]

Hint: split(',') breaks string into list by commas [OK]

Common Mistakes:

• Expecting a string instead of list
• Using split() without argument
• Confusing split() with join()

4. The following code is intended to replace all spaces with dashes in the string text. What is the error?

text = "hello world"
text.replace(" ", "-")
print(text)

medium

A. No error, output is 'hello-world'

B. RuntimeError because replace() needs assignment

C. Output is 'hello world' because replace() does not change original string

D. SyntaxError due to wrong replace() usage

Solution

1. Step 1: Understand string immutability in Python

   Strings cannot be changed in place; methods like replace() return a new string.

2. Step 2: Check code behavior

   text.replace(" ", "-") returns new string but original text remains unchanged because result is not assigned.

3. Final Answer:

   Output is 'hello world' because replace() does not change original string -> Option C

4. Quick Check:

   replace() returns new string, assign it [OK]

Hint: Assign replace() result to variable to update string [OK]

Common Mistakes:

• Not assigning replace() result
• Expecting replace() to modify string in place
• Confusing syntax errors with logic errors

5. You have a list of words: words = [' apple', 'Banana ', ' CHERRY ']. Which code correctly creates a new list with all words trimmed and in lowercase?

hard

A. [word.strip() + word.lower() for word in words]

B. [word.strip().lower() for word in words]

C. [word.lower().strip for word in words]

D. [word.lower() for word in words.strip()]

Solution

1. Step 1: Understand the order of strip() and lower()

   strip() removes spaces, lower() converts to lowercase. Order doesn't matter here since they don't interfere.

2. Step 2: Analyze each option

   A concatenates strip() result with lower() result, doubling content. B calls lower() then references .strip without (), yielding method objects. C chains strip().lower() correctly. D calls strip() on list, causing AttributeError.

3. Final Answer:

   [word.strip().lower() for word in words] -> Option B

4. Quick Check:

   List comprehension with strip() and lower() [OK]

Hint: Use list comprehension with strip() then lower() [OK]

Common Mistakes:

• Concatenating strings instead of chaining methods
• Calling strip() on list
• Forgetting parentheses on strip()