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Intermediate Python > 2. Python File I/O (14m) > Flashcards

2. Python File I/O (14m) Flashcards

1
Q

Explain the following code:

def remember(thing):
    # open file
    file = open("database.txt", "a")
    # write thing to file
    file.write(thing+"\n")
    # close file
    file.close()

if \_\_name\_\_ == '\_\_main\_\_':
    remember(input("What should I remember? "))

& Explain the difference of the following code to the first one:
~~~
def remember(thing):
# open file
with open(“database.txt”, “a”) as file:
# write thing to file
file.write(thing+”\n”)

if __name__ == ‘__main__’:
remember(input(“What should I remember? “))
~~~

A

Certainly! Here’s a detailed explanation of each line of the code:

  1. def remember(thing):
    • This line defines a function named remember that takes one parameter called thing. It is a function definition.
  2. file = open("database.txt", "a")
    • This line opens a file named “database.txt” in “append” mode ("a"). It assigns the file object to the variable file. This line is responsible for opening the file.
  3. file.write(thing+"\n")
    • This line writes the value of the thing parameter to the file. The thing value is concatenated with "\n" to add a newline character after the text. It appends the content to the file since we opened it in “append” mode.
  4. file.close()
    • This line closes the file that was opened in the previous line. It’s important to close the file to release system resources and ensure that all data is written and saved properly.
  5. if \_\_name\_\_ == '\_\_main\_\_':
    • This line checks if the script is being run directly (as opposed to being imported as a module). The \_\_name\_\_ variable contains the name of the current module, and when the script is the main module being executed, its value will be '\_\_main\_\_'.
  6. remember(input("What should I remember? "))
    • This line prompts the user for input by displaying the message “What should I remember? “. The input() function reads a line of text from the user. The entered value is passed as an argument to the remember() function, which was defined earlier. This line invokes the remember() function, passing the user’s input as the thing argument.

In summary, the code defines a function remember() that opens a file in “append” mode, writes a given input to the file, and then closes the file. When the script is run directly, it prompts the user for input and uses the remember() function to save the input to the “database.txt” file.

The two code snippets are similar in their purpose of writing a given input to a file called “database.txt” in append mode. However, there is a difference in how the file is handled and closed.

In the first code snippet:
~~~
def remember(thing):
# open file
file = open(“database.txt”, “a”)
# write thing to file
file.write(thing+”\n”)
# close file
file.close()

if __name__ == ‘__main__’:
remember(input(“What should I remember? “))
~~~
The file is opened using the open function and assigned to the variable file. The input thing is written to the file using the write method of the file object. After writing, the file is explicitly closed using the close method.

In the second code snippet:
~~~
def remember(thing):
# open file
with open(“database.txt”, “a”) as file:
# write thing to file
file.write(thing+”\n”)

if __name__ == ‘__main__’:
remember(input(“What should I remember? “))
~~~
The file is opened using the open function and assigned to the variable file within a with statement. The with statement automatically takes care of closing the file once the block is exited, regardless of whether an exception occurs or not. This eliminates the need for an explicit file.close() call.

The key difference between the two approaches is that the first code snippet relies on explicitly closing the file using file.close(), which can be prone to errors if the file is not closed properly. The second code snippet, on the other hand, uses a with statement, which ensures that the file is properly closed even if an exception occurs within the block.

Overall, the second code snippet with the with statement is considered a more Pythonic and safer way to handle file operations as it takes care of closing the file automatically, reducing the risk of resource leaks and improving code readability.

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2
Q

What does import sys do in Python?

A

In Python, the import sys statement is used to import the sys module into your program. The sys module provides access to various system-specific parameters and functions, allowing interaction with the Python interpreter and the underlying operating system.

By importing the sys module, you gain access to functionality such as:

  1. System-specific parameters: The sys module provides variables that contain information related to the Python interpreter and the environment in which it is running. For example, sys.argv provides access to the command-line arguments passed to the script, sys.path contains a list of directories that Python searches for modules, and sys.version stores the Python interpreter’s version information.
  2. Standard I/O streams: The sys module gives you access to the standard input (sys.stdin), standard output (sys.stdout), and standard error (sys.stderr) streams. You can use these streams to read input from the user, write output to the console, or handle error messages.
  3. Exiting the program: The sys module provides the sys.exit() function, which allows you to exit the program with a specified exit status. It’s commonly used to terminate the program execution under certain conditions or in response to an error.
  4. Runtime environment manipulation: The sys module offers functions like sys.getsizeof() to retrieve the size of objects in memory, sys.getrecursionlimit() and sys.setrecursionlimit() to get or set the maximum recursion depth, and sys.settrace() to set a function for debugging and profiling purposes.

These are just a few examples of what you can do with the sys module. It provides a wide range of functionalities for interacting with the system and the Python interpreter, making it a powerful tool for system-related tasks and program control.

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3
Q

Explain the following code:

import sys

def remember(thing):
    # open file
    with open("database.txt", "a") as file:   
        # write thing to file
        file.write(thing+"\n")
        
def show():
    # open file
    with open("database.txt", "r") as file:
    		# print out each line in file
        for line in file:
            print(line)

if \_\_name\_\_ == '\_\_main\_\_':
    if sys.argv[1].lower() == '--list':
        show()
    else:
    		remember(' '.join(sys.argv[1:]))
A

Certainly! Here’s a more detailed explanation of each line of the code:

```python
import sys
~~~
The line import sys imports the sys module, which provides access to system-specific parameters and functions. It allows us to interact with the system, access command-line arguments, and perform other system-related tasks.

```python
def remember(thing):
~~~
This line defines a function named remember that takes a single parameter called thing. The function is designed to store the value of thing in a file.

```python
with open(“database.txt”, “a”) as file:
~~~
This line opens the file named “database.txt” in append mode using the open function. The file object is assigned to the variable file. The with statement is used to handle the opening and closing of the file automatically. It ensures that the file is properly closed after the indented block of code.

```python
file.write(thing + “\n”)
~~~
This line writes the value of the thing parameter, concatenated with a newline character ("\n"), to the opened file using the write method of the file object. The "\n" adds a newline at the end of each entry, ensuring that each entry is written on a separate line in the file.

```python
def show():
~~~
This line defines a function named show with no parameters. The purpose of this function is to display the contents of the file on the console.

```python
with open(“database.txt”, “r”) as file:
~~~
This line opens the “database.txt” file in read mode using the open function. The file object is assigned to the variable file. Similar to before, the with statement is used to handle the file opening and closing automatically.

```python
for line in file:
~~~
This line starts a loop that iterates over each line in the opened file. It reads the file line by line.

```python
print(line)
~~~
This line prints each line from the file to the console using the print function. It displays the content of the file on separate lines.

```python
if __name__ == ‘__main__’:
~~~
This line checks if the script is being executed as the main program.

```python
if sys.argv[1].lower() == ‘–list’:
~~~
This line checks if the second command-line argument (index 1) passed to the script is equal to the string '--list'. The sys.argv list contains the command-line arguments passed to the script. sys.argv[0] represents the script name itself, so sys.argv[1] corresponds to the first argument provided.

```python
show()
~~~
If the condition is true, this line calls the show function. It means that if the second command-line argument is --list, the show function will be executed, displaying the contents of the file.

```python
else:
~~~
If the condition in the previous if statement is false, this line indicates the alternative path of execution.

```python
remember(‘ ‘.join(sys.argv[1:]))
~~~
This line calls the remember function with a string argument obtained by joining all the command-line arguments (excluding the script name) using a space as the separator. The sys.argv[1:] list slicing excludes the script name from the arguments. The join method concatenates the remaining arguments with spaces between them, creating a single string.

Overall, this code allows you to append new entries to the “database.txt” file using the remember function and display the contents of the

file using the show function. The script checks the command-line argument provided. If it is --list, it shows the file contents; otherwise, it stores the arguments as entries in the file.

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4
Q

Explain the following:

teachers.txt file contains the following:
~~~
Nick Pettit
Amit Bijlani
Pasan Premaratne
Ben Jakuben
Guil Hernandez
Andrew Chalkley
Dave McFarland
Kenneth Love
Craig Dennis
Huston Hedinger
Gabe Nadel
Jeremy McLain
Ben Deitch
Chris Ramacciotti
Alena Holligan
Carling Kirk
James Churchill
~~~

In REPL or interactive python, the following code is run one by one:
~~~
file = open(‘teachers.txt’)
file.read(10)
file.seek(0)
file.read(15)
file.seek(0)
file.read(30)
lines = file.readlines()
len(lines)
lines
for line in lines:
print(line[::-1])
~~~

Explain each line of above code with what results each will give.

A

Let’s break down each line of the code and explain the results it will produce:

```python
file = open(‘teachers.txt’)
~~~
This line opens the file named ‘teachers.txt’ in the current directory and assigns the file object to the variable file. The file is opened in the default read mode (‘r’).

```python
file.read(10)
~~~
This line reads the next 10 characters from the current position in the file. Since the file was just opened, it will read the first 10 characters. The returned value will be a string containing those 10 characters.

```python
file.seek(0)
~~~
This line moves the file position indicator back to the beginning of the file (offset 0). It resets the file position to the start, allowing us to read the file again from the beginning.

```python
file.read(15)
~~~
This line reads the next 15 characters from the current position in the file. Since the file position was reset to the beginning, it will read the first 15 characters of the file. The returned value will be a string containing those 15 characters.

```python
file.seek(0)
~~~
This line again moves the file position indicator back to the beginning of the file, resetting it to the start.

```python
file.read(30)
~~~
This line reads the next 30 characters from the current position in the file. Since the file position was reset to the beginning, it will read the first 30 characters of the file. The returned value will be a string containing those 30 characters.

```python
lines = file.readlines()
~~~
This line reads all the remaining lines from the current position in the file and returns them as a list of strings. Each line of the file becomes an element in the list.

```python
len(lines)
~~~
This line returns the number of elements (lines) in the lines list, which corresponds to the number of lines in the file.

```python
lines
~~~
This line prints the lines list, which contains all the lines of the file as separate elements.

```python
for line in lines:
print(line[::-1])
~~~
This loop iterates over each line in the lines list. For each line, it reverses the line using slicing (line[::-1]) and prints the reversed line. This effectively prints each line of the file in reverse order.

Overall, the code reads and manipulates the contents of the ‘teachers.txt’ file. It demonstrates various file reading operations such as reading characters, seeking to different positions, reading lines, and printing the contents of the file.

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5
Q

Explain the following Treehouse Notes in detail with examples:

  1. open(filename, mode="r") opens a file. More info in the docs.
  2. file.read(bytes=-1) would read the entire contents of the file. You can control the number of bytes read by passing in an integer. Relatedly, file.seek() will move the read/write pointer to another part of the file.
  3. file.readlines() reads the entire file into a list, with each line as a list item.

The context manager pattern for dealing with files is:

```python
with open(“my_file.txt”, “r”) as file:
file.read(10)
~~~

For more about sys.argv, check out the docs.

A

Certainly! Let’s explain each of the Treehouse Notes in detail:

  1. open(filename, mode="r") opens a file. More info in the docs.This function, open(), is used to open a file in Python. It takes at least one argument, which is the filename representing the name of the file to be opened. Additionally, you can specify the mode parameter to control the purpose of opening the file. The default mode is "r" (read mode), which allows you to read the contents of the file.Examples:
    python
file = open("data.txt", "r")  # Opens the file "data.txt" in read mode

    In the example above, the file named “data.txt” is opened in read mode, and the file object is assigned to the variable file.
  2. file.read(bytes=-1) would read the entire contents of the file. You can control the number of bytes read by passing in an integer. Relatedly, file.seek() will move the read/write pointer to another part of the file.Once you have opened a file and obtained a file object, you can use the read() method to read the contents of the file. By default, calling file.read() with no arguments reads the entire contents of the file.Examples:
    python
content = file.read()  # Reads the entire contents of the file

    The read() method reads the entire contents of the file and assigns it to the variable content.However, you can control the number of bytes read by passing an integer as an argument to read(). This allows you to read a specific number of bytes from the file.Examples:
    python
content = file.read(100)  # Reads the next 100 bytes from the file

    In this example, read(100) reads the next 100 bytes from the file and assigns them to the variable content.The file.seek() method is used to change the current position of the file read/write pointer. It allows you to move the pointer to a specific position in the file, enabling you to read from or write to different parts of the file.Examples:
    python
file.seek(0)  # Moves the file pointer to the beginning of the file
content = file.read(50)  # Reads the next 50 bytes from the file

    In this example, seek(0) moves the file pointer to the beginning of the file, and then read(50) reads the next 50 bytes from that position.
  3. file.readlines() reads the entire file into a list, with each line as a list item.The readlines() method is used to read all the lines of a file and store them as individual elements in a list. Each line of the file becomes a separate string item in the list.Examples:
    python
lines = file.readlines()  # Reads all lines of the file into a list

    The readlines() method reads all the lines of the file and assigns them to the variable lines.You can then iterate over the lines list to access each line individually.Example:
    python
for line in lines:
    print(line)

    This example prints each line of the file on a separate line in the console.
  4. The context manager pattern for dealing with files is:```python
    with open(“my_file.txt”, “r”) as file:
    file.read(

10)
```

The context manager pattern is a recommended way to work with files in Python. It ensures that the file is properly opened and closed, even if exceptions occur during the execution.

In the example above, the with statement is used with the open() function to open the file “my_file.txt” in read mode. The file object is assigned to the variable file. The indented block of code under the with statement can access the file object and perform operations on the file.

Once the block of code is executed or if an exception occurs, the file is automatically closed. This eliminates the need for an explicit file.close() call and helps prevent resource leaks.

  1. For more about sys.argv, check out the docs.The sys.argv variable in Python contains a list of command-line arguments passed to the script. It allows you to access and process the arguments provided when executing the Python script from the command line.To learn more about sys.argv and its usage, you can refer to the official Python documentation or online resources that provide detailed information on working with command-line arguments in Python.
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Intermediate Python (9 decks)

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