Logic

Introducing logic

Sometimes you want to answer a question with some logic rather than directly asking the user for information.

Docassemble uses Python, a powerful but easy to read programming language to control logic inside an interview. Python statements go inside a code block.

---
code: |
  # This is a comment. notice that we assign a value with =, and we test a value with ==
  if user_name == "Scotty":
    secret_message = "Beam Me Up, Scotty"
  else:
    secret_message = "No message to report."
---    
question: |
  Hello, ${user_name}
subquestion: |
  ${secret_message}
mandatory: True
---
question: |
  What is your name?
fields:
  - Name: user_name

In the example above, we introduced the use of the | line continuation marker, or vertical pipe. We always use this when the text that follows could go on multiple lines, and to handle special characters (like accented letters). You also always use it for a code block.

We also introduced the subquestion specifier. subquestion is used for longer explanation text on a screen. It uses a smaller font that is not bold.

Sometimes you don't want to use logic to create a new variable. You can use simple Python code right inside a question or subquestion block, although the syntax is very slightly different.

---  
question: Hello, ${user_name}
subquestion: |
  % if user_name == 'Spock':
  Live long and prosper.
  % endif
mandatory: True
---
question: What is your name?
fields:
  - Name: user_name

Notice that inside a question block, the line with the if statement starts with %. We also need to use an endif statement, instead of using indentation to show the beginning and end of the if statement.

Note: YAML is picky about indentation. If you run into an error, check to make sure each line is indented the same way as the example above.

Python allows us to go a little further than using just if-then. We can introduce multiple branches using elif:

---
code: |
  # This is a comment. notice that we assign a value with =, and we test a value with ==
  if user_name == "Scotty":
    secret_message = "Beam Me Up, Scotty"
  elif user_name == "Bones":
    secret_message = "Dammit Jim!"
  else:
    secret_message = "No message to report."
---    
question: |
  Hello, ${user_name}
subquestion: |
  ${secret_message}
mandatory: True
---
question: |
  What is your name?
fields:
  - Name: user_name

Note: once a condition is satisfied in an if-then block of logic, Python moves on and stops the evaluation process. What do you think the result of this would be if the user puts "Bones" as their name?

---
code: |
  # This is a comment. notice that we assign a value with =, and we test a value with ==
  if user_name == "Scotty":
    secret_message = "Beam Me Up, Scotty"
  elif user_name == "Bones":
    secret_message = "Dammit Jim!"
  elif user_name == "Bones":
    secret_message = "Here is the antidote!"
  else:
    secret_message = "No message to report."
---    
question: |
  Hello, ${user_name}
subquestion: |
  ${secret_message}
mandatory: True
---
question: |
  What is your name?
fields:
  - Name: user_name

Your assignment

1. Modify the Logic exercise so that a new secret message is displayed when a name of your choice is displayed.
2. Stretch goal: make all of the secret messages work regardless of how the user capitalizes their name. Hint: using the .lower() method of a Python string should help you out.

Quinten Steenhuis, June 2020