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Reading and writing binary files

From: Python 3 Essential Training

Video: Reading and writing binary files

Python makes a distinction between text files and binary files. Even on operating systems with file systems that don't make that distinction, Python still does. So how you read and write binary files, even though it's still very simple, is different in significant ways from how you read and write text files. So let's make a working copy of files.py. files-working.py. Open that working file and we see we have our little loop here that reads lines of text and prints them to the screen, and let's just go ahead and take this out and put in olives.jpg and we will save that and run it, and we will see that we get this UnicodeDecodeError because Python is trying to decode the text in that JPEG file.

Reading and writing binary files

Python makes a distinction between text files and binary files. Even on operating systems with file systems that don't make that distinction, Python still does. So how you read and write binary files, even though it's still very simple, is different in significant ways from how you read and write text files. So let's make a working copy of files.py. files-working.py. Open that working file and we see we have our little loop here that reads lines of text and prints them to the screen, and let's just go ahead and take this out and put in olives.jpg and we will save that and run it, and we will see that we get this UnicodeDecodeError because Python is trying to decode the text in that JPEG file.

That's a JPEG file. If we open that up, we will see there is an image of some grapes on a vine there, and that it not a text file at all. So what we need to do is we need to open this in binary mode. So we are going to open it in Read Binary, like that. And now if we save this and run it, we are going to get this output, which is still not really what we want. What's happening is that the Print function is trying to print that in a way that's text readable, and we are not going to want to write that to a file if we are making a copy of this.

So what we need to do is we need to open an output file in binary mode and we need to use our buffered I/O. So we will call this infile, and we will open an outfile, and call that new.jpg. Open that in write binary mode, and we are going to do a buffered I/O here. So we will start by giving ourselves a buffersize. Let's see, how big is this file? Properties, it's 142K.

So we will just go ahead and make our buffersize 50000, and that will work fine. And the rest of it is very similar to how we would do this with the text file, infile.read(buffersize) and infile.read is not an iterable, so we have to use a while loop, length of buffer, and we will outfile.write from our buffer.

And we would go ahead and print a dot. So we see that something is happening on the screen and we will read the next buffer. When we are all done, we will print a blank line and we will print the word Done. You can see that we are done. So what's different here is that by opening the file in binary mode, we are no longer dealing with text.

The rest of it looks pretty much the same as how we do the buffered reading and writing with a text file, but the difference here is that we are not working with text at all. This buffer is now a binary object. It's not a text object at all. We will go ahead and we will save this and we will run it and we have got a few little dots there. And if we refresh our file system, which Eclipse does not do for us, we have this new.jpg. I open that up. There is our JPEG intact. So we know that copy worked.

I will look at the size of it. It's the right number of bytes, 142309. Is that the same as our olives? It is exactly the same. So we have an exact duplicate of that file. So reading and writing binary files, the methodology is very similar, except you have to use the buffered I/O. You are not going to want to use line oriented I/O for a binary file. And most of the time for text files, you are going to use line oriented I/O, although you can use buffered I/O for text files as well.

As a matter of fact, you can use binary mode for text files if you want to, and deal with them as bytes, and that will certainly work. But the distinction here is that with binary files, you have to use the buffered I/O and you have to use binary data types. So when you read that file, it's going to read it as an array of bytes and it's not going to read it as text. So this is how you do buffered I/O with binary files in Python. And it's really very simple and it's something that you are going to use now and then as you are dealing with binary files.

One final note. You will notice that the buffered read method is not an utterable. If I was going to be doing this a lot, I would go ahead and I would write a method for an object that is iterable. That would make this easier for me to do. It's certainly a matter of style. It's something that you might think about doing if you are going to be doing this a lot, and there is an example of how to make a generator function that generates an iterable in both the Functions chapter and the Classes chapter in this course. So that's how you do buffered I/O on binary files in Python.

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This video is part of

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Python 3 Essential Training

87 video lessons · 42178 viewers

Bill Weinman
Author

 
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  1. 5m 14s
    1. Welcome
      1m 32s
    2. Understanding prerequisites for Python
      2m 4s
    3. Using the exercise files
      1m 38s
  2. 33m 29s
    1. Getting started with "Hello World"
      4m 43s
    2. Selecting code with conditionals
      4m 45s
    3. Repeating code with a loop
      4m 13s
    4. Reusing code with a function
      2m 43s
    5. Creating sequences with generator functions
      2m 46s
    6. Reusing code and data with a class
      4m 39s
    7. Greater reusability with inheritance and polymorphism
      7m 17s
    8. Handling errors with exceptions
      2m 23s
  3. 22m 32s
    1. Installing Python 3 and Eclipse for Windows
      11m 24s
    2. Installing Python 3 and Eclipse for Mac
      11m 8s
  4. 28m 0s
    1. Creating a main script
      3m 27s
    2. Understanding whitespace in Python
      4m 8s
    3. Commenting code
      3m 28s
    4. Assigning values
      3m 37s
    5. Selecting code and values with conditionals
      4m 46s
    6. Creating and using functions
      3m 54s
    7. Creating and using objects
      4m 40s
  5. 31m 23s
    1. Understanding variables and objects in Python
      2m 46s
    2. Distinguishing mutable and immutable objects
      2m 41s
    3. Using numbers
      3m 34s
    4. Using strings
      6m 38s
    5. Aggregating values with lists and tuples
      4m 55s
    6. Creating associative lists with dictionaries
      4m 24s
    7. Finding the type and identity of a variable
      4m 45s
    8. Specifying logical values with True and False
      1m 40s
  6. 9m 42s
    1. Selecting code with if and else conditional statements
      2m 22s
    2. Setting multiple choices with elif
      2m 14s
    3. Understanding other strategies for multiple choices
      2m 38s
    4. Using the conditional expression
      2m 28s
  7. 11m 26s
    1. Creating loops with while
      1m 27s
    2. Iterating with for
      3m 54s
    3. Enumerating iterators
      3m 22s
    4. Controlling loop flow with break, continue, and else
      2m 43s
  8. 23m 28s
    1. Performing simple arithmetic
      2m 14s
    2. Operating on bitwise values
      3m 30s
    3. Comparing values
      3m 32s
    4. Operating on Boolean values
      2m 59s
    5. Operating on parts of a container with the slice operator
      6m 52s
    6. Understanding operator precedence
      4m 21s
  9. 11m 34s
    1. Using the re module
      1m 4s
    2. Searching with regular expressions
      3m 12s
    3. Replacing with regular expressions
      3m 29s
    4. Reusing regular expressions with re.compile
      3m 49s
  10. 9m 10s
    1. Learning how exceptions work
      1m 18s
    2. Handling exceptions
      4m 15s
    3. Raising exceptions
      3m 37s
  11. 23m 1s
    1. Defining functions
      6m 23s
    2. Using lists of arguments
      2m 26s
    3. Using named function arguments
      4m 32s
    4. Returning values from functions
      1m 55s
    5. Creating a sequence with a generator function
      7m 45s
  12. 47m 29s
    1. Understanding classes and objects
      5m 12s
    2. Using methods
      6m 12s
    3. Using object data
      10m 4s
    4. Understanding inheritance
      5m 11s
    5. Applying polymorphism to classes
      7m 13s
    6. Using generators
      9m 48s
    7. Using decorators
      3m 49s
  13. 18m 54s
    1. Understanding strings as objects
      3m 25s
    2. Working with common string methods
      5m 24s
    3. Formatting strings with str.format
      5m 31s
    4. Splitting and joining strings
      2m 49s
    5. Finding and using standard string methods
      1m 45s
  14. 25m 27s
    1. Creating sequences with tuples and lists
      4m 6s
    2. Operating on sequences with built-in methods
      5m 50s
    3. Organizing data with dictionaries
      4m 56s
    4. Operating on character data with bytes and byte arrays
      10m 35s
  15. 11m 46s
    1. Opening files
      2m 4s
    2. Reading and writing text files
      4m 33s
    3. Reading and writing binary files
      5m 9s
  16. 21m 27s
    1. Creating a database with SQLite 3
      6m 56s
    2. Creating, retrieving, updating, and deleting records
      7m 31s
    3. Creating a database object
      7m 0s
  17. 18m 27s
    1. Using standard library modules
      8m 0s
    2. Finding third-party modules
      5m 47s
    3. Creating a module
      4m 40s
  18. 23m 11s
    1. Dealing with syntax errors
      8m 19s
    2. Dealing with runtime errors
      4m 0s
    3. Dealing with logical errors
      4m 22s
    4. Using unit tests
      6m 30s
  19. 19m 56s
    1. Normalizing a database interface
      6m 39s
    2. Deconstructing a database application
      8m 9s
    3. Displaying random entries from a database
      5m 8s
  20. 29s
    1. Goodbye
      29s

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