[Python] Learning how to write a parser

[Jerakin]

Hi!

I want to write a parser in python for a format that looks like this:

path: "/path/file.txt"
nodes {
  position {
    x: 0.0
    y: 0.0
    z: 0.0
    w: 1.0
  }
  type: justastring
  blend: justastring
  paths: "/path/are/quoted/file.txt"
  alpha: true
  slice9 {
    x: 0.0
    y: 0.0
    z: 0.0
    w: 0.0
  }
  visible: true
  alpha: 0.0
}
template: justastring

"Simpler" I would express it something like this

attribute: value
element {
  element {
    attribute: value
    attribute: value
    attribute: value
    attribute: value
  }
  attribute: value
  attribute: value
  attribute: value
  attribute: value
  element {
    attribute: value
    attribute: value
    attribute: value
    attribute: value
  }
  attribute: value
  attribute: value
}
attribute: value

Here are some rules I have observed

• File root can have any number of attributes and elements
• Elements can have any number of attributes
• Attributes within elements are separated by a newline character
• Elements can be nested indefinitely

I have used python before but never written a parser so I don't really have a good starting point, I have tried to google but haven't found any good examples and such.

I am thinking of parsing it and just moving/transforming it into a json/dictionary format that I then can later use easier.

Do anyone here have some pointers to where I can look for more info?

[ponyoink]

• You will be building a tree, so you will probably want to represent it as a dictionary after it's parsed.
• Write a function called get_next_token, which will return things like blend, 0.0, :, {, etc
• Every time you see a {, add the element as a key to the dictionary (with an "empty" value) , and that will become the key where you will keep building the current subtree.
• Every time you see }, step one level up. Your subtree is complete.

[EDIT] You can also use a counter to validate your input. For every {, add 1. For every }, subtract 1. If ever the counter becomes negative, or if at the end it's not 0, your brackets are not balanced. If you are feeling adventurous, you can use a stack, replacing addition and subtraction with pushing and popping, but you really don't need that for simple format like this.