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Reading and writing text files

From: Python 3 Essential Training

Video: Reading and writing text files

Reading and writing text files in Python is really very simple. Let's make a working copy of files.py. files-working.py. Open that working copy and we see that we have the simple loop here that reads lines of a file and prints them to the screen. So if we run that, we see there is our lines of file, print it to the screen, and here is our input file. So, we can see that that's exactly the same. Now, if we wanted to, instead of bringing that to the screen, if we wanted to write it to a file, we could do that.

Reading and writing text files

Reading and writing text files in Python is really very simple. Let's make a working copy of files.py. files-working.py. Open that working copy and we see that we have the simple loop here that reads lines of a file and prints them to the screen. So if we run that, we see there is our lines of file, print it to the screen, and here is our input file. So, we can see that that's exactly the same. Now, if we wanted to, instead of bringing that to the screen, if we wanted to write it to a file, we could do that.

Simply change this to infile, so we can have a separate one for outfile, outfile equals open and we'll call this new.txt. We will open that for write. And we will open this one here for read. It defaults to read but if I am going to specify write, then I am going to want to also specify read. This is called infile, and all we have to do here is to say file equals outfile, And now that print is going to print the outfile instead of to the screen.

And as we don't have anything coming to the screen, I am just going to print done here. So we know that we have gotten to this point. We get some feedback on the screen. I will save that and I will run it and it says done. If I refresh the file system here, which Eclipse does not do by itself, you can see there is a new.txt and it's got our five lines of text in it. So that is just a very simple way to write to a text file. Obviously, it doesn't have to be something you are reading from another text file.

You can generate your own whatever and write it out to a text file very easily, using print and the File parameter for print. Let's say that what we are doing is not necessarily line oriented. Let's say that maybe we have a very large file with thousands of lines of text, And we don't really need to read them and write them line by line. We want to do it in some bigger chunks. So I have a file here called bigfile.txt and you will notice it is a really big file of text. It has 10,000 lines of text in it.

If we look at the properties, we can see that it's 320,000 bytes. It's 300k. So, let's go ahead and open that one instead, and call it bigfile.txt and we are going to read this in what's called buffered mode, instead of doing it line by line. We will set a buffer size and I will say that's going to be 50,000 bytes. It's 300,000 bytes, so it will give us a few chunks and then we are going to start by filling our buffer, infile.read and buffersize and then instead of this here, I am going to use a While loop, because the Read method on the file handle object is not an iterable.

So we can't use a For loop, so we are going to use a While loop. We are going to say while the length of buffer. And so as long as our buffer is not empty then we will outfile.write buffer. That's really all there is to it. We read it again in the loop, so that we are not just writing the same buffer forever. So I am going to write buffer = infile.read buffersize and after we do the write, let's just print a dot to the screen, so we have some feedback that we are doing something, dot, end, equals nothing.

So this will just print dots on the screen right next to each other, and we are all done. Before the Done, we are going to want to print a blank line. So we will go ahead and save that and run it and we see we have got a handful of dots there. It means that we have written something and we refresh our file system and take a look at the properties of new.txt and it's exactly 320,000 bytes. We will open that up and there is all of our lines of text in there. So what we have done here is we simply read it in a few big chunks and obviously 50,000 bytes is very small by today's standard.

Your buffers can be bigger than that or if you are on a mobile device or something small, maybe you do want them that small. We just did that this way for illustration, but this shows you how you can read and write in buffered I/O mode. So it allows you to deal with bigger chunks of files than just line by line. So that's how simple it is to read and write text files in Python and we will discuss reading and writing binary files in a separate lesson.

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This video is part of

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Python 3 Essential Training

87 video lessons · 43011 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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