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Iterating with for

From: Python 3 Essential Training

Video: Iterating with for

One of the most common reasons for using a loop in programming is what's called an iterator. This is where you need to step through some data structure, taking one object at a time, and doing something with each of those objects before going back for the next one. Let's take a look at how this works in Python with the 'for' loop. We'll make a working copy of for.py, and call it for-working.py, and we'll go ahead and open that working copy. Here, we are opening a file with lines of text and reading that file line-by- line and printing out each line on the screen.

Iterating with for

One of the most common reasons for using a loop in programming is what's called an iterator. This is where you need to step through some data structure, taking one object at a time, and doing something with each of those objects before going back for the next one. Let's take a look at how this works in Python with the 'for' loop. We'll make a working copy of for.py, and call it for-working.py, and we'll go ahead and open that working copy. Here, we are opening a file with lines of text and reading that file line-by- line and printing out each line on the screen.

So, let's go ahead and run it and see what it does, and we see that it prints these lines of text, and there they are. If we open the file here, lines.txt, we see that it has these five lines of text. You'll notice that in the file the lines are right next to each other, and in our result, it's printing a blank line between them. The reason for that is that the print function prints a blank line after each object that it prints, and the line itself has a line ending at the end of it. So, we can make print not to do that by typing end = " " just a empty string. If we save that and run it, then we get the lines of text, just like we expect them.

So, the readlines method, inside the file object, is what's called an iterator. What this does is it takes one object at a time from a sequence of objects, and it returns those, one at a time, in this case into the line variable. The for loop is designed to work with iterators, so the for keyword introduces the for loop, and then a variable name, which is used to contain each object in the iteration, and in keyword introduces the iterator.

So, it reads like for line in iteration or for line in file object readlines, and then it prints each line, one at a time. So, this here, this file object, .readlines, is the iterator. In Python, all the container types are iterators. So, if I have a list, let's say 1, 2, 3, 4, 5, then it will work exactly the same. It will just print 1, 2, 3, 4, 5 all in one line because we are not ending our lines with a new line here in the print function.

So, if I save that and run it, we just get 12345. In fact, a string is a container object, and this will work exactly the same way. It will just take each letter from the string, and of course that will just look like the word string. But instead of we take this out and print them one line at a time, we can see that we are getting each letter as separate object. So, the string is a container that contains individual characters. In this case, we are printing the lines of the file, and the readlines method, from the file object, is the iterator that gives us one line at a time.

So, if I save this and run it, there we get the lines of the file. So, that's the for loop. The for loop is used for stepping through iterators, and virtually all container types in Python are iterators. So, you'll see the for loop use a lot. In fact, in my informal survey of Python code, as I was preparing for this course, I saw probably 20 or 30 for loops for every while loop that I saw. The for loop is by far the most common type of a loop in Python because iteration is such a common thing to need to do, and so many objects in Python are iterators.

So, that's the for loop, and that's how it works for line in readlines, so it's for variable in container. Then that variable becomes available to the suite of the loop, and that's how a for loop works in Python.

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This video is part of

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Python 3 Essential Training

87 video lessons · 38683 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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