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Selecting code and values with conditionals

From: Python 3 Essential Training

Video: Selecting code and values with conditionals

There are two different types of conditionals in Python. There is conditional execution, and there is conditional values. Let's take a look at what these look like. We'll make a working copy of syntax.py, call it syntax-conditionals.py. Go ahead and open our working copy, and we'll start in here by defining a couple of variables. We'll just call them a and b, and give them values of 0 and 1.

Selecting code and values with conditionals

There are two different types of conditionals in Python. There is conditional execution, and there is conditional values. Let's take a look at what these look like. We'll make a working copy of syntax.py, call it syntax-conditionals.py. Go ahead and open our working copy, and we'll start in here by defining a couple of variables. We'll just call them a and b, and give them values of 0 and 1.

And then we'll say if a < b, and we'll print a is less than b. We can get rid of this line here. So what we have here - I'm going to save that - is an assignment for assigning two values, 0 and 1, two variables, a and b. And then we're testing if a is less than b, and so this is a conditional statement.

And it has colon here, and that colon means there is going to be a suite, or a block of code, that is indented under that level. And so here we have just a print statement, so that's the only statement in that suite, and it will only print if a is actually less than b. So if I save this and run it, we get a is less than b in our result. If I change this 0 here to a 1, then we will get nothing in our result.

It won't print anything, you see. There's nothing there. Now, I can handle that exceptional case, and I can say else: print(a is not less than b), and save that and run it, and now we see a is not less than b because, in fact a is equal to b. So this is the common if and else in Python. You also have an else if, which in Python is spelled like this, elif.

And you need another condition, else if a > b, and we'll print a is greater than b, and then our else. For else, we know that if it's not less than and if it's not greater than then it is equal, a = b. So, if we save this and run it, of course we get a = b, and if we make the a a larger number than the b, and save that and run it, then we get a > b.

So, this is the conditional execution form of conditionals in Python. We have the if clause, we have the else if, and you can have a string of else ifs, if you want to. And we have the else, for if the case where none of the ifs and else ifs are evaluated to true. There is another form of conditional in Python, which is the conditional expression, or the conditional value. So we'll start with our same assignment (a, b = 0, 1). We'll say (s = "less than" if a < b else "not less than) and then we'll go ahead and print(s).

So what this does is this is a conditional expression. We have a value that's getting assigned only if this condition is true; otherwise, this other value would be assigned. You may have seen this in other languages done with question marks and colons, and the designers of Python felt that that was too obscure and too difficult to read and decided instead to use syntax that's much easier to read, and in fact, this is much easier to read.

It's very clear from just looking at this that (s = "less than" if a < b else "not less than), and otherwise, we'll say "not less than". Let's go ahead and save it and run it, and we see that we get the word less than, and if I make this a 1, now it's now less that anymore, and I'll run that, and it says "not less than". So that is the conditional value, or the conditional expression in Python. So Python has two different types of conditionals. It's got conditional Execution, which is with the traditional if, elif, and else, and it has conditional expressions, which don't use the question mark and colon syntax of some other C-based languages, which instead use the same keywords if and else. It is actually a whole lot easier to read.

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This video is part of

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Python 3 Essential Training

87 video lessons · 38638 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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