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Contracts

caution
You are reading documentation for an outdated version. Here’s the latest one!

Python is a dynamically typed language, but each of our methods expects its arguments to be of specific types and value ranges. To avoid wrong inputs, the tracker takes two approaches:

  1. It specifies type hints on each function argument and return value. This allows IDEs to validate function calls in your code and also enables static type checking using tools such as mypy. However, type violations do not result in runtime errors.
  2. It validates expected value ranges during runtime and throws ValueError errors in case they are violated. Contracts are defined for arguments to check that they belong to expected value ranges such as:
s = Subject()
tracker.set_platform("coffee") # throws ValueError because "coffee" is not one of supported platforms
tracker.set_screen_resolution(width=-1, height=-1) # throws ValueError because width and height arguments have to be greater than 0

You can turn off runtime contract validation like this:

from snowplow_tracker import disable_contracts
disable_contracts()

Contracts in tracker version 0.9.1​

Earlier versions of the tracker up to v0.9.1 used the PyContracts library to validate argument types and value ranges. In contrast with the latest versions, pycontracts was also used to validate argument types, raising ContractNotRespected errors at runtime when passing values with wrong types (this behavior was dropped in favor of type hints in Python 3):

s = Subject()
tracker.set_color_depth("walrus")
contracts.interface.ContractNotRespected: Breach for argument 'depth' to Subject:set_color_depth().
Expected type 'int', got 'str'.
checking: Int      for value: Instance of str: 'walrus'
checking: $(Int)   for value: Instance of str: 'walrus'
checking: int      for value: Instance of str: 'walrus'
Variables bound in inner context:
- self: Instance of Tracker: <snowplow_tracker.tracker.Tracker object...> [clip]

If your value is of the wrong type, convert it before passing it into the track...() method, for example:

level_idx = 42
tracker.track_screen_view("Game Level", str(level_idx))

You can turn off type checking to improve performance like this:

from snowplow_tracker import disable_contracts
disable_contracts()