Searching and replacing text in Python - Step-by-Step Execution

Concept Flow - Searching and replacing text

Start with original text

↓

Search for target substring

↓

Is target found?

No→End, return original text

Yes↓

Replace target with new substring

↓

Return modified text

↓

End

The program starts with the original text, searches for a target substring, replaces it if found, and returns the modified text.

Execution Sample

Python

text = "Hello world"
new_text = text.replace("world", "Python")
print(new_text)

This code replaces the word 'world' with 'Python' in the text and prints the result.

Execution Table

Step	Action	Evaluation	Result
1	Set text variable	text = "Hello world"	text = "Hello world"
2	Call replace method	text.replace("world", "Python")	"Hello Python"
3	Assign replaced text to new_text	new_text = "Hello Python"	new_text = "Hello Python"
4	Print new_text	print(new_text)	Output: Hello Python
5	End of program	No more code	Program ends

💡 Program ends after printing the replaced text.

Variable Tracker

Variable	Start	After replace	Final
text	"Hello world"	"Hello world"	"Hello world"
new_text	undefined	"Hello Python"	"Hello Python"

Key Moments - 2 Insights

Why does the original 'text' variable not change after replace?

Because the replace method returns a new string and does not modify the original string. See execution_table step 2 and variable_tracker where 'text' stays the same.

What happens if the target substring is not found?

The replace method returns the original string unchanged. The program would print the original text. This is implied in the concept_flow where 'No' branch returns original text.

Visual Quiz - 3 Questions

Look at the execution_table at step 2, what is the result of the replace method?

A "Hello world"

B "Hello Python"

C "Python world"

D "Hello"

💡 Hint

Check the 'Result' column in execution_table row with Step 2.

At which step is the new_text variable assigned the replaced string?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Action' column in execution_table where new_text is assigned.

If the target substring 'world' was not in text, what would new_text be after replace?

A "Hello Python"

B "Python"

C "Hello world"

D "" (empty string)

💡 Hint

Recall that replace returns original string if target not found, see key_moments explanation.

Concept Snapshot

Searching and replacing text in Python:
- Use str.replace(old, new) to replace substrings.
- It returns a new string; original string stays unchanged.
- If old substring not found, original string is returned.
- Assign result to a new variable or overwrite.
- Example: new_text = text.replace("old", "new")

Full Transcript

This example shows how to search and replace text in Python using the replace method. The program starts with a string variable 'text' containing 'Hello world'. It calls text.replace("world", "Python") which returns a new string 'Hello Python'. This new string is assigned to 'new_text'. The original 'text' variable remains unchanged because strings are immutable in Python. Finally, the program prints 'Hello Python'. If the target substring is not found, replace returns the original string unchanged. This process is shown step-by-step in the execution table and variable tracker.

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