File modes and access types in Python - Step-by-Step Execution

Concept Flow - File modes and access types

Open file with mode

↓

Check mode type

↓

Read data

↓

Close file after operation

Open a file with a mode, then read, write, or append data accordingly, and finally close the file.

Execution Sample

Python

f = open('test.txt', 'w')
f.write('Hello')
f.close()

Open a file for writing, write 'Hello' to it, then close the file.

Execution Table

Step	Action	File Mode	Operation	Result
1	Open file	'w'	Open for writing	File created or emptied
2	Write data	'w'	Write 'Hello'	Data written to file
3	Close file	'w'	Close file	File saved and closed
4	End	-	-	File closed, operations complete

💡 File closed after writing, no more operations allowed without reopening

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
f	None	File object opened in 'w' mode	File object with 'Hello' written	File object closed

Key Moments - 3 Insights

Why does opening a file in 'w' mode erase existing content?

Because 'w' mode means write and it clears the file first, as shown in execution_table step 1 where the file is created or emptied.

Can you read from a file opened in 'w' mode?

No, 'w' mode only allows writing. Reading would cause an error, as the mode restricts operations (see execution_table step 2).

What happens if you forget to close the file?

The file might not save changes properly or lock resources. Closing (step 3) ensures data is saved and resources freed.

Visual Quiz - 3 Questions

Look at the execution_table, what is the file mode used when writing 'Hello'?

A 'a'

B 'w'

C 'r'

D 'x'

💡 Hint

Check step 2 in execution_table where writing happens with mode 'w'

At which step is the file closed according to the execution_table?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for the 'Close file' action in execution_table

If the file was opened in 'a' mode instead of 'w', what would change in the execution_table?

A Data would be appended, not overwrite

B File would be emptied at step 1

C File would be opened for reading

D File would not be closed

💡 Hint

Refer to concept_flow where 'a' means append data

Concept Snapshot

File modes control how you open files:
'r' = read only (file must exist)
'w' = write only (creates or empties file)
'a' = append (add data to end)
Always close files to save changes and free resources.

Full Transcript

This lesson shows how to open files in Python using different modes. Opening a file with 'w' mode means you can write to it, but it erases any existing content first. You write data using the write() method, then close the file to save changes. Opening with 'r' lets you read data, but you cannot write. Opening with 'a' lets you add data to the end without erasing. Always close files after operations to avoid problems. The execution table traces opening, writing, and closing steps with the file object variable changing state accordingly.

Practice

1. Which file mode in Python opens a file for reading only, and raises an error if the file does not exist?
file = open('data.txt', mode)

easy

A. "r"

B. "w"

C. "a"

D. "x"

Solution

1. Step 1: Understand the purpose of mode "r"

   Mode "r" opens a file for reading only and requires the file to exist.

2. Step 2: Compare with other modes

   Mode "w" opens for writing (creates or truncates), "a" appends, and "x" creates a new file but errors if it exists.

3. Final Answer:

   "r" -> Option A

4. Quick Check:

   Read-only mode = "r" [OK]

Hint: Read-only mode is just "r" [OK]

Common Mistakes:

• Confusing "r" with "w" which overwrites files
• Using "a" thinking it reads
• Choosing "x" which creates files

2. Which of the following is the correct syntax to open a file named log.txt for appending text in Python?

easy

A. open('log.txt', 'r')

B. open('log.txt', 'w')

C. open('log.txt', 'a')

D. open('log.txt', 'x')

Solution

1. Step 1: Identify the mode for appending

   Mode "a" opens the file for appending, adding new content at the end.

2. Step 2: Check syntax correctness

   The syntax open('log.txt', 'a') is correct for appending text.

3. Final Answer:

   open('log.txt', 'a') -> Option C

4. Quick Check:

   Append mode = "a" [OK]

Hint: Append mode is always "a" [OK]

Common Mistakes:

• Using "r" which is read-only
• Using "w" which overwrites the file
• Using "x" which creates new file only

3. What will be the output of the following code if example.txt contains the text "Hello"?

with open('example.txt', 'w') as f:
    f.write('World')

with open('example.txt', 'r') as f:
    print(f.read())

medium

A. World

B. Hello

C. HelloWorld

D. Error

Solution

1. Step 1: Understand mode "w" effect on file content

   Opening with "w" overwrites the file, so "World" replaces "Hello".

2. Step 2: Reading the file after writing

   Reading the file after writing will output the new content "World".

3. Final Answer:

   World -> Option A

4. Quick Check:

   Write mode overwrites content = "World" [OK]

Hint: Write mode "w" replaces file content [OK]

Common Mistakes:

• Assuming "w" appends instead of overwrites
• Expecting original content to remain
• Thinking reading causes error after writing

4. What is wrong with this code snippet?

f = open('data.txt', 'r')
content = f.read()
f.write('More data')
f.close()

medium

A. No error, code is correct

B. File is not closed properly

C. File mode should be "a" to read

D. File is opened in read mode but write is attempted

Solution

1. Step 1: Check file mode and operations

   The file is opened in "r" (read) mode, which does not allow writing.

2. Step 2: Identify the error cause

   Calling f.write() in read mode causes a runtime error because writing is not allowed.

3. Final Answer:

   File is opened in read mode but write is attempted -> Option D

4. Quick Check:

   Write not allowed in "r" mode [OK]

Hint: Write needs "w" or "a" mode, not "r" [OK]

Common Mistakes:

• Forgetting write is disallowed in read mode
• Not closing file properly (though here it is closed)
• Confusing append mode with read mode

5. You want to create a new file report.txt but only if it does not already exist. Which mode should you use to avoid overwriting existing files?

hard

A. "w"

B. "x"

C. "a"

D. "r+"

Solution

1. Step 1: Understand mode "x" behavior

   Mode "x" creates a new file and raises an error if the file already exists, preventing overwriting.

2. Step 2: Compare with other modes

   "w" overwrites, "a" appends or creates, "r+" opens for reading and writing but requires file to exist.

3. Final Answer:

   "x" -> Option B

4. Quick Check:

   Create new only = "x" mode [OK]

Hint: Use "x" to create file only if not exists [OK]

Common Mistakes:

• Using "w" which overwrites existing files
• Using "a" which appends or creates silently
• Using "r+" which needs existing file