Searching and replacing text in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What Python method is commonly used to replace parts of a string?

The str.replace(old, new) method is used to replace occurrences of old substring with new substring in a string.

beginner

How do you search for a substring inside a string in Python?

You can use the in keyword or the str.find() method. in returns True or False, while find() returns the index or -1 if not found.

intermediate

What does str.replace('a', 'b', 1) do?

It replaces only the first occurrence of 'a' with 'b' in the string. The third argument limits the number of replacements.

intermediate

Why might you use the re.sub() function instead of str.replace()?

re.sub() allows replacing text using patterns (regular expressions), which is more powerful for complex searching and replacing.

beginner

What will this code output?

text = 'apple apple apple'
print(text.replace('apple', 'orange', 2))

It will output: orange orange apple because only the first two 'apple' words are replaced.

Which method replaces all occurrences of a substring in Python?

A str.split(separator)

B str.replace(old, new)

C str.index(substring)

D str.find(substring)

str.replace() replaces all occurrences of old with new.

What does str.find(substring) return if the substring is not found?

A -1

B 0

C None

D Raises an error

str.find() returns -1 if the substring is not found.

How can you replace only the first occurrence of a substring in Python?

A Use <code>str.split()</code>

B Use <code>str.replace(old, new)</code>

C Use <code>str.find()</code>

D Use <code>str.replace(old, new, 1)</code>

The third argument in str.replace() limits the number of replacements.

Which Python module provides advanced searching and replacing with patterns?

A re

B os

C sys

D math

The re module supports regular expressions for pattern matching.

What will this code print?

text = 'cat dog cat'
print(text.replace('cat', 'fox', 1))

A cat dog fox

B fox dog fox

C fox dog cat

D cat dog cat

Only the first 'cat' is replaced with 'fox'.

Explain how to search for a substring and replace it in a Python string.

Think about how to check if text exists and then how to change it.

Describe when and why you would use the re.sub() function instead of str.replace().

Consider cases where simple text replacement is not enough.

Practice

1. What does the replace method do in Python strings?

easy

A. It finds a specified substring and replaces it with another substring.

B. It deletes a substring from the string permanently.

C. It changes the original string directly without creating a new one.

D. It counts how many times a substring appears in the string.

Solution

1. Step 1: Understand the purpose of replace()

   The replace method searches for a substring and replaces it with another substring.

2. Step 2: Check if it modifies original string

   Strings are immutable in Python, so replace returns a new string without changing the original.

3. Final Answer:

   It finds a specified substring and replaces it with another substring. -> Option A

4. Quick Check:

   replace() = find and change substring [OK]

Hint: Remember: replace returns new string, original stays same [OK]

Common Mistakes:

• Thinking replace changes the original string
• Confusing replace with delete or count methods
• Assuming replace modifies in-place

2. Which of the following is the correct syntax to replace all occurrences of 'cat' with 'dog' in a string text?

easy

A. text.replace('cat', 'dog', all=True)

B. text.replace('cat', 'dog')

C. text.replace('cat' -> 'dog')

D. text.replace('cat', 'dog', count=0)

Solution

1. Step 1: Recall replace() syntax

   The correct syntax is string.replace(old, new[, count]), where old and new are strings.

2. Step 2: Analyze options

   text.replace('cat', 'dog') matches the correct syntax. Options A, B, and D use invalid syntax or parameters.

3. Final Answer:

   text.replace('cat', 'dog') -> Option B

4. Quick Check:

   replace(old, new) syntax = text.replace('cat', 'dog') [OK]

Hint: Use replace(old, new) without extra keywords [OK]

Common Mistakes:

• Using arrows or keywords inside replace()
• Adding unsupported parameters like all=True
• Confusing parameter order

3. What will be the output of this code?

text = 'apple apple apple'
new_text = text.replace('apple', 'orange', 2)
print(new_text)

medium

A. 'apple orange orange'

B. 'orange orange orange'

C. 'orange orange apple'

D. 'apple apple orange'

Solution

1. Step 1: Understand replace with count

   The third argument 2 limits replacements to first two occurrences.

2. Step 2: Apply replacements

   First two 'apple' become 'orange', last remains 'apple'. Result: 'orange orange apple'.

3. Final Answer:

   'orange orange apple' -> Option C

4. Quick Check:

   replace with count=2 changes first two only [OK]

Hint: Count limits how many replacements happen [OK]

Common Mistakes:

• Replacing all occurrences ignoring count
• Replacing from the end instead of start
• Miscounting number of replacements

4. The following code tries to replace 'blue' with 'red' but causes an error. What is the error?

text = 'blue sky'
text.replace('blue', 'red', 'all')
print(text)

medium

A. SyntaxError due to wrong quotes

B. AttributeError because replace is not a string method

C. No error, prints 'red sky'

D. TypeError because 'all' is not an integer for count

Solution

1. Step 1: Check replace() parameters

   The third parameter must be an integer count, but 'all' is a string.

2. Step 2: Identify error type

   Passing a string instead of int causes a TypeError at runtime.

3. Final Answer:

   TypeError because 'all' is not an integer for count -> Option D

4. Quick Check:

   replace count must be int, not string [OK]

Hint: Count parameter must be integer, not string [OK]

Common Mistakes:

• Using strings instead of integers for count
• Expecting replace to modify original string
• Confusing error types

5. You have a paragraph stored in text. You want to replace only the first 3 occurrences of the word 'error' with 'issue'. Which code snippet correctly does this?

hard

A. new_text = text.replace('error', 'issue', 3)

B. new_text = text.replace('error', 'issue', count=3)

C. new_text = text.replace('error', 'issue', '3')

D. new_text = text.replace('error', 'issue')[:3]

Solution

1. Step 1: Use replace with count parameter

   To replace only first 3 occurrences, use replace(old, new, count) with count=3.

2. Step 2: Analyze options

   new_text = text.replace('error', 'issue', 3) uses correct syntax. new_text = text.replace('error', 'issue', count=3) uses invalid keyword argument. new_text = text.replace('error', 'issue', '3') passes count as string. new_text = text.replace('error', 'issue')[:3] slices string incorrectly.

3. Final Answer:

   new_text = text.replace('error', 'issue', 3) -> Option A

4. Quick Check:

   replace(old, new, 3) replaces first 3 only [OK]

Hint: Use third argument as integer count for limited replacements [OK]

Common Mistakes:

• Using keyword count instead of positional argument
• Passing count as string instead of int
• Trying to slice string after replace