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Assert Module

Overview

ytt’s Assert module allows users to make assertions about their templates and stop execution if desired.

Functions

The @ytt:assert module provides several built-in assertion functions. To use these functions, include the @ytt:assert module:

#@ load("@ytt:assert", "assert")

assert.equals()

  • Checks equality of the two arguments provided, stops execution if values are not equal.
load("@ytt:assert", "assert")

assert.equals("not", "equal") # stops execution

assert.fail()

  • Stops execution and reports a failure.
  • Takes a single string argument used as the failure message, this can be formatted with available values.
load("@ytt:assert", "assert")

assert.fail("custom failure message")
assert.fail("expected value foo, but was {}".format(value))
x = data.values.env.mysql_password or assert.fail("missing env.mysql_password")

assert.max()

  • Checks that values are less than or equal to the maximum value.
  • x = assert.max(4) returns an object, x, that can x.check() if values are less than or equal to 4.
  • Is able to compare numbers, strings, lists, dictionaries, and YAML fragments.
load("@ytt:assert", "assert")

assert.max(4).check(3)

assert.max(4).check(5) # stops execution

assert.max_len()

  • Checks that values have length less than or equal to the maximum length.
  • Maximum length argument is an integer.
  • x = assert.max_len(4) creates an object, x that can check() if the length of values are less than or equal to 4.
  • Checks the length of strings, lists, dictionaries, and YAML fragments.
load("@ytt:assert", "assert")

assert.max_len(4).check({'foo': 0, 'bar': 1})

assert.max(4)_len.check("123.45.67.89") # stops execution

assert.min()

  • Checks that values are greater than or equal to the minimum value.
  • x = assert.min(2) returns an object, x, that can x.check() if values are greater than or equal to 2.
  • Is able to compare numbers, strings, lists, dictionaries, and YAML fragments.
load("@ytt:assert", "assert")

assert.min(2).check(5)

assert.min(2).check(1) # stops execution

assert.min_len()

  • Checks that values have length greater than or equal to the minimum length.
  • Minimum length argument is an integer.
  • x = assert.min_len(1) creates an object, x that can check() if the length of values are greater than or equal to 1.
  • Checks the length of strings, lists, dictionaries, and YAML fragments.
load("@ytt:assert", "assert")

assert.min_len(1).check(["some","list","of","values"])

assert.min_len(1).check("") # stops execution

assert.not_null()

  • Checks that a value is not null or none.
load("@ytt:assert", "assert")

v = None
assert.not_null(v) # stops execution
# is syntactic sugar for
assert.not_null().check(v) # stops execution

assert.one_not_null()

  • Checks that a map (or dictionary)’s value has one and only one not-null item.
load("@ytt:assert", "assert")

# passes
assert.one_not_null().check({"foo": 1, "bar": None})

# fails: two values are not null (stops execution)
assert.one_not_null().check({"foo": 1, "bar": 2})

# passes: one of named values is not null
assert.one_not_null(["foo", "bar"]).check({"foo": 1, "bar": None, "baz": 3})

# passes: missing keys are ok
assert.one_not_null(["foo", "not-present"]).check({"foo": 1, "bar": 2})

assert.one_of()

  • Checks that the value is one in the specified list
load("@ytt:assert", "assert")

# passes
assert.one_of(["debug", "info", "warn"]).check("warn")

# fails: value not in the list.
assert.one_of(["aws", "azure", "gcp"]).check("digitalocean")

# An assertion can be used multiple times against different values
valid_ports = assert.one_of([1433, 1434, 1521, 1830, 3306, 5432])

# all pass
valid_ports.check(3306)
valid_ports.check(5432)

# fails: items in enumeration are integers, value is a string
valid_ports.check("5432")

assert.try_to()

  • Invokes a function, catching failure if one occurs.
  • Takes single function as argument.
  • Returns the return value of function, or the error if one occurs.
load("@ytt:assert", "assert")

x, err = assert.try_to(lambda : json.decode('{"key": "value"}'))
x     # { "key" = "value" }    (i.e. dict with one entry)
err   # None

x, err = assert.try_to(lambda : json.decode("(not JSON)"))
x     # None
err   # "json.decode: invalid character '(' looking for beginning of value"
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