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1# This file is part of Hypothesis, which may be found at
2# https://github.com/HypothesisWorks/hypothesis/
3#
4# Copyright the Hypothesis Authors.
5# Individual contributors are listed in AUTHORS.rst and the git log.
6#
7# This Source Code Form is subject to the terms of the Mozilla Public License,
8# v. 2.0. If a copy of the MPL was not distributed with this file, You can
9# obtain one at https://mozilla.org/MPL/2.0/.
11from hypothesis.internal.reflection import get_pretty_function_description
12from hypothesis.strategies._internal.strategies import (
13 SampledFromStrategy,
14 SearchStrategy,
15 T,
16 is_simple_data,
17)
18from hypothesis.strategies._internal.utils import cacheable, defines_strategy
21class JustStrategy(SampledFromStrategy):
22 """A strategy which always returns a single fixed value.
24 It's implemented as a length-one SampledFromStrategy so that all our
25 special-case logic for filtering and sets applies also to just(x).
27 The important difference from a SampledFromStrategy with only one
28 element to choose is that JustStrategy *never* touches the underlying
29 choice sequence, i.e. drawing neither reads from nor writes to `data`.
30 This is a reasonably important optimisation (or semantic distinction!)
31 for both JustStrategy and SampledFromStrategy.
32 """
34 @property
35 def value(self):
36 return self.elements[0]
38 def __repr__(self):
39 suffix = "".join(
40 f".{name}({get_pretty_function_description(f)})"
41 for name, f in self._transformations
42 )
43 if self.value is None:
44 return "none()" + suffix
45 return f"just({get_pretty_function_description(self.value)}){suffix}"
47 def calc_is_cacheable(self, recur):
48 return is_simple_data(self.value)
50 def do_filtered_draw(self, data):
51 # The parent class's `do_draw` implementation delegates directly to
52 # `do_filtered_draw`, which we can greatly simplify in this case since
53 # we have exactly one value. (This also avoids drawing any data.)
54 return self._transform(self.value)
57@defines_strategy(never_lazy=True)
58def just(value: T) -> SearchStrategy[T]:
59 """Return a strategy which only generates ``value``.
61 Note: ``value`` is not copied. Be wary of using mutable values.
63 If ``value`` is the result of a callable, you can use
64 :func:`builds(callable) <hypothesis.strategies.builds>` instead
65 of ``just(callable())`` to get a fresh value each time.
67 Examples from this strategy do not shrink (because there is only one).
68 """
69 return JustStrategy([value])
72@defines_strategy(force_reusable_values=True)
73def none() -> SearchStrategy[None]:
74 """Return a strategy which only generates None.
76 Examples from this strategy do not shrink (because there is only
77 one).
78 """
79 return just(None)
82class Nothing(SearchStrategy):
83 def calc_is_empty(self, recur):
84 return True
86 def do_draw(self, data):
87 # This method should never be called because draw() will mark the
88 # data as invalid immediately because is_empty is True.
89 raise NotImplementedError("This should never happen")
91 def calc_has_reusable_values(self, recur):
92 return True
94 def __repr__(self):
95 return "nothing()"
97 def map(self, f):
98 return self
100 def filter(self, f):
101 return self
103 def flatmap(self, f):
104 return self
107NOTHING = Nothing()
110@cacheable
111@defines_strategy(never_lazy=True)
112def nothing() -> SearchStrategy:
113 """This strategy never successfully draws a value and will always reject on
114 an attempt to draw.
116 Examples from this strategy do not shrink (because there are none).
117 """
118 return NOTHING
121class BooleansStrategy(SearchStrategy):
122 def do_draw(self, data):
123 return data.draw_boolean()
125 def __repr__(self):
126 return "booleans()"