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14.1 Reading from files

2 min read•june 24, 2024

Python's file input/output capabilities allow you to read from and write to files on your computer. This powerful feature enables you to work with external data, store program results, and manage persistent information across program executions.

The function is the gateway to file operations in Python. By understanding different file modes and reading methods, you can efficiently handle various file types and sizes, while proper closing practices ensure resource management and data integrity.

File Input/Output in Python

File opening with open()

Use the
```
open()
```
function to open a file and obtain a
- Syntax:
```
file_object = open(file_path, mode)
```
  - ```
  file_path
```
  represents the path to the file as a string (
```
  'data.txt'
```
  )
- ```
mode
```
    specifies how the file should be opened, default is ' for read mode (
```
'r'
```
    ,
```
'[rb'](https://www.fiveableKeyTerm:rb')
```
    )
Common read modes:
- 'r': read mode (default), opens the file for reading only, at the beginning
- 'rb': binary read mode, used for reading non-text files (images, audio)

Ensure the file exists and has read permissions to avoid

FileNotFoundError

[PermissionError](https://www.fiveableKeyTerm:PermissionError)

Consider the (absolute or relative) when opening files

Reading methods for file contents

```
[read()](https://www.fiveableKeyTerm:read())
```
reads the entire contents of a file as a single string
- Syntax:
```
file_contents = file_object.read()
```
- Returns an empty string when reaching the end of the file
```
[readline()](https://www.fiveableKeyTerm:readline())
```
reads a single line from the file
- Syntax:
```
line = file_object.readline()
```
- Returns an empty string at the end of the file
- Useful for reading large files line by line to avoid memory issues
```
[readlines()](https://www.fiveableKeyTerm:readlines())
```
reads all lines of a file and returns them as a list of strings
- Syntax:
```
lines = file_object.readlines()
```
- Each line becomes an element in the list (
```
['line1\n', 'line2\n']
```
  )
- Useful for processing file contents line by line
File pointer advances after each read operation, subsequent reads continue from the current position
Be aware of when reading lines from a file

Proper file closing practices

Always close files after reading or writing to free up system resources
- Syntax:
```
file_object.close()
```
Closing a file ensures:
- All data is written to the file (for write operations)
- File resources are released back to the operating system
- Other programs can access the file without conflicts

Use a

[try-finally](https://www.fiveableKeyTerm:try-finally)

block to ensure the file is always closed, even if an exception occurs

Syntax:

try:
    file_object = open(file_path, mode)
    # Perform file operations
finally:
    file_object.close()

Alternatively, use a

with

statement () to automatically close the file after the block

Syntax:

with open(file_path, mode) as file_object:
    # Perform file operations

Additional File Operations

: Use
```
file_object.seek()
```
to move the file pointer to a specific position
: Control how data is buffered when reading or writing files
: Specify the text encoding when opening files (e.g., UTF-8, ASCII)

Key Terms to Review (17)

Buffering: Buffering is a technique used in computer systems to temporarily store data in a buffer, which is a reserved area of memory, before it is processed or transmitted. This helps manage the flow of data and ensures that information is not lost or corrupted during input/output operations or file reading/writing processes.

Context Manager: A context manager is a Python object that provides a way to manage the setup and teardown of resources, ensuring that resources are properly initialized and cleaned up, even in the face of errors or exceptions. It is a powerful tool for working with files, network connections, and other resources that require explicit initialization and cleanup.

Encoding: Encoding is the process of converting information from one format or representation into another, often to facilitate storage, transmission, or processing of data. It plays a crucial role in the context of reading from files and working with files in different locations, including CSV files.

File Object: A file object is a Python data structure that represents an open file on the computer's file system. It provides a set of methods and attributes that allow you to read, write, and manipulate the contents of the file.

File path: A file path is a string that specifies the unique location of a file or directory in a file system. It provides a way to navigate to the file, whether it’s on your local machine or on a network. Understanding file paths is crucial for reading from files, as they help you identify where your files are stored and how to access them accurately.

File Pointer: A file pointer is a reference to the current position within an open file. It keeps track of where the program is reading from or writing to in the file, allowing for efficient and controlled access to the file's contents.

File Seek: File seek is a fundamental operation in file I/O that allows you to position the file pointer at a specific location within a file. It enables you to read from or write to a specific part of the file, rather than just the beginning or end.

Newline Characters: Newline characters are special characters used to represent the end of a line of text or the start of a new line. They are an essential component in the context of reading from and writing to files, as they help maintain the proper formatting and structure of the data being processed.

Open(): The $open()$ function in Python is used to open a file and returns a corresponding file object. It allows for various modes such as reading, writing, and appending.

PermissionError: PermissionError is an exception that occurs when a program attempts to access a file or resource without the necessary permissions. This error can arise in various contexts, including when reading from or writing to files, as well as when working with files in different locations or CSV files.

R': The 'r' prefix in Python is used to create a raw string literal, which treats backslashes as literal characters rather than escape characters. This is particularly useful when working with file paths and regular expressions, where backslashes are commonly used as special characters.

Rb': The 'rb' mode in Python is used to open a file in read-binary mode, which allows you to read the file's contents as raw bytes rather than as text. This mode is useful when working with non-text files, such as images, audio, or other binary data.

Read(): The 'read()' function is a built-in Python function used to read data from a file. It allows you to access and retrieve the contents of a file, which can then be processed, analyzed, or manipulated as needed within your Python program.

Readline(): readline() is a built-in function in Python that allows you to read a single line of input from the user or from a file. It is a powerful tool for interacting with data in Python, particularly in the context of reading from files.

Readlines(): readlines() is a built-in function in Python that reads all the lines from a file and returns them as a list of strings. Each string represents a single line from the file, including the newline character at the end of each line. This function is particularly useful when you need to process the contents of a file line by line.

Try-finally: The try-finally statement in Python is a control flow structure that ensures a block of code is executed regardless of whether an exception is raised or not. The 'try' block contains the code that may raise an exception, while the 'finally' block contains the code that must be executed regardless of the outcome of the 'try' block.

With statement: A with statement is a control flow structure in Python that simplifies exception handling and resource management by ensuring proper acquisition and release of resources, such as files. It automatically handles the closing of a file once the block of code within it is executed, making it easier to manage files and handle exceptions without requiring explicit cleanup code.

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