# Source code for _pytest.python_api

```
import math
import pprint
from collections.abc import Sized
from decimal import Decimal
from numbers import Complex
from types import TracebackType
from typing import Any
from typing import Callable
from typing import cast
from typing import Generic
from typing import Iterable
from typing import List
from typing import Mapping
from typing import Optional
from typing import overload
from typing import Pattern
from typing import Sequence
from typing import Tuple
from typing import Type
from typing import TYPE_CHECKING
from typing import TypeVar
from typing import Union
if TYPE_CHECKING:
from numpy import ndarray
import _pytest._code
from _pytest.compat import final
from _pytest.compat import STRING_TYPES
from _pytest.outcomes import fail
def _non_numeric_type_error(value, at: Optional[str]) -> TypeError:
at_str = f" at {at}" if at else ""
return TypeError(
"cannot make approximate comparisons to non-numeric values: {!r} {}".format(
value, at_str
)
)
def _compare_approx(
full_object: object,
message_data: Sequence[Tuple[str, str, str]],
number_of_elements: int,
different_ids: Sequence[object],
max_abs_diff: float,
max_rel_diff: float,
) -> List[str]:
message_list = list(message_data)
message_list.insert(0, ("Index", "Obtained", "Expected"))
max_sizes = [0, 0, 0]
for index, obtained, expected in message_list:
max_sizes[0] = max(max_sizes[0], len(index))
max_sizes[1] = max(max_sizes[1], len(obtained))
max_sizes[2] = max(max_sizes[2], len(expected))
explanation = [
f"comparison failed. Mismatched elements: {len(different_ids)} / {number_of_elements}:",
f"Max absolute difference: {max_abs_diff}",
f"Max relative difference: {max_rel_diff}",
] + [
f"{indexes:<{max_sizes[0]}} | {obtained:<{max_sizes[1]}} | {expected:<{max_sizes[2]}}"
for indexes, obtained, expected in message_list
]
return explanation
# builtin pytest.approx helper
class ApproxBase:
"""Provide shared utilities for making approximate comparisons between
numbers or sequences of numbers."""
# Tell numpy to use our `__eq__` operator instead of its.
__array_ufunc__ = None
__array_priority__ = 100
def __init__(self, expected, rel=None, abs=None, nan_ok: bool = False) -> None:
__tracebackhide__ = True
self.expected = expected
self.abs = abs
self.rel = rel
self.nan_ok = nan_ok
self._check_type()
def __repr__(self) -> str:
raise NotImplementedError
def _repr_compare(self, other_side: Any) -> List[str]:
return [
"comparison failed",
f"Obtained: {other_side}",
f"Expected: {self}",
]
def __eq__(self, actual) -> bool:
return all(
a == self._approx_scalar(x) for a, x in self._yield_comparisons(actual)
)
def __bool__(self):
__tracebackhide__ = True
raise AssertionError(
"approx() is not supported in a boolean context.\nDid you mean: `assert a == approx(b)`?"
)
# Ignore type because of https://github.com/python/mypy/issues/4266.
__hash__ = None # type: ignore
def __ne__(self, actual) -> bool:
return not (actual == self)
def _approx_scalar(self, x) -> "ApproxScalar":
if isinstance(x, Decimal):
return ApproxDecimal(x, rel=self.rel, abs=self.abs, nan_ok=self.nan_ok)
return ApproxScalar(x, rel=self.rel, abs=self.abs, nan_ok=self.nan_ok)
def _yield_comparisons(self, actual):
"""Yield all the pairs of numbers to be compared.
This is used to implement the `__eq__` method.
"""
raise NotImplementedError
def _check_type(self) -> None:
"""Raise a TypeError if the expected value is not a valid type."""
# This is only a concern if the expected value is a sequence. In every
# other case, the approx() function ensures that the expected value has
# a numeric type. For this reason, the default is to do nothing. The
# classes that deal with sequences should reimplement this method to
# raise if there are any non-numeric elements in the sequence.
pass
def _recursive_list_map(f, x):
if isinstance(x, list):
return [_recursive_list_map(f, xi) for xi in x]
else:
return f(x)
class ApproxNumpy(ApproxBase):
"""Perform approximate comparisons where the expected value is numpy array."""
def __repr__(self) -> str:
list_scalars = _recursive_list_map(self._approx_scalar, self.expected.tolist())
return f"approx({list_scalars!r})"
def _repr_compare(self, other_side: "ndarray") -> List[str]:
import itertools
import math
def get_value_from_nested_list(
nested_list: List[Any], nd_index: Tuple[Any, ...]
) -> Any:
"""
Helper function to get the value out of a nested list, given an n-dimensional index.
This mimics numpy's indexing, but for raw nested python lists.
"""
value: Any = nested_list
for i in nd_index:
value = value[i]
return value
np_array_shape = self.expected.shape
approx_side_as_list = _recursive_list_map(
self._approx_scalar, self.expected.tolist()
)
if np_array_shape != other_side.shape:
return [
"Impossible to compare arrays with different shapes.",
f"Shapes: {np_array_shape} and {other_side.shape}",
]
number_of_elements = self.expected.size
max_abs_diff = -math.inf
max_rel_diff = -math.inf
different_ids = []
for index in itertools.product(*(range(i) for i in np_array_shape)):
approx_value = get_value_from_nested_list(approx_side_as_list, index)
other_value = get_value_from_nested_list(other_side, index)
if approx_value != other_value:
abs_diff = abs(approx_value.expected - other_value)
max_abs_diff = max(max_abs_diff, abs_diff)
if other_value == 0.0:
max_rel_diff = math.inf
else:
max_rel_diff = max(max_rel_diff, abs_diff / abs(other_value))
different_ids.append(index)
message_data = [
(
str(index),
str(get_value_from_nested_list(other_side, index)),
str(get_value_from_nested_list(approx_side_as_list, index)),
)
for index in different_ids
]
return _compare_approx(
self.expected,
message_data,
number_of_elements,
different_ids,
max_abs_diff,
max_rel_diff,
)
def __eq__(self, actual) -> bool:
import numpy as np
# self.expected is supposed to always be an array here.
if not np.isscalar(actual):
try:
actual = np.asarray(actual)
except Exception as e:
raise TypeError(f"cannot compare '{actual}' to numpy.ndarray") from e
if not np.isscalar(actual) and actual.shape != self.expected.shape:
return False
return super().__eq__(actual)
def _yield_comparisons(self, actual):
import numpy as np
# `actual` can either be a numpy array or a scalar, it is treated in
# `__eq__` before being passed to `ApproxBase.__eq__`, which is the
# only method that calls this one.
if np.isscalar(actual):
for i in np.ndindex(self.expected.shape):
yield actual, self.expected[i].item()
else:
for i in np.ndindex(self.expected.shape):
yield actual[i].item(), self.expected[i].item()
class ApproxMapping(ApproxBase):
"""Perform approximate comparisons where the expected value is a mapping
with numeric values (the keys can be anything)."""
def __repr__(self) -> str:
return "approx({!r})".format(
{k: self._approx_scalar(v) for k, v in self.expected.items()}
)
def _repr_compare(self, other_side: Mapping[object, float]) -> List[str]:
import math
approx_side_as_map = {
k: self._approx_scalar(v) for k, v in self.expected.items()
}
number_of_elements = len(approx_side_as_map)
max_abs_diff = -math.inf
max_rel_diff = -math.inf
different_ids = []
for (approx_key, approx_value), other_value in zip(
approx_side_as_map.items(), other_side.values()
):
if approx_value != other_value:
max_abs_diff = max(
max_abs_diff, abs(approx_value.expected - other_value)
)
max_rel_diff = max(
max_rel_diff,
abs((approx_value.expected - other_value) / approx_value.expected),
)
different_ids.append(approx_key)
message_data = [
(str(key), str(other_side[key]), str(approx_side_as_map[key]))
for key in different_ids
]
return _compare_approx(
self.expected,
message_data,
number_of_elements,
different_ids,
max_abs_diff,
max_rel_diff,
)
def __eq__(self, actual) -> bool:
try:
if set(actual.keys()) != set(self.expected.keys()):
return False
except AttributeError:
return False
return super().__eq__(actual)
def _yield_comparisons(self, actual):
for k in self.expected.keys():
yield actual[k], self.expected[k]
def _check_type(self) -> None:
__tracebackhide__ = True
for key, value in self.expected.items():
if isinstance(value, type(self.expected)):
msg = "pytest.approx() does not support nested dictionaries: key={!r} value={!r}\n full mapping={}"
raise TypeError(msg.format(key, value, pprint.pformat(self.expected)))
class ApproxSequencelike(ApproxBase):
"""Perform approximate comparisons where the expected value is a sequence of numbers."""
def __repr__(self) -> str:
seq_type = type(self.expected)
if seq_type not in (tuple, list, set):
seq_type = list
return "approx({!r})".format(
seq_type(self._approx_scalar(x) for x in self.expected)
)
def _repr_compare(self, other_side: Sequence[float]) -> List[str]:
import math
import numpy as np
if len(self.expected) != len(other_side):
return [
"Impossible to compare lists with different sizes.",
f"Lengths: {len(self.expected)} and {len(other_side)}",
]
approx_side_as_map = _recursive_list_map(self._approx_scalar, self.expected)
number_of_elements = len(approx_side_as_map)
max_abs_diff = -math.inf
max_rel_diff = -math.inf
different_ids = []
for i, (approx_value, other_value) in enumerate(
zip(approx_side_as_map, other_side)
):
if approx_value != other_value:
abs_diff = abs(approx_value.expected - other_value)
max_abs_diff = max(max_abs_diff, abs_diff)
if other_value == 0.0:
max_rel_diff = np.inf
else:
max_rel_diff = max(max_rel_diff, abs_diff / abs(other_value))
different_ids.append(i)
message_data = [
(str(i), str(other_side[i]), str(approx_side_as_map[i]))
for i in different_ids
]
return _compare_approx(
self.expected,
message_data,
number_of_elements,
different_ids,
max_abs_diff,
max_rel_diff,
)
def __eq__(self, actual) -> bool:
try:
if len(actual) != len(self.expected):
return False
except TypeError:
return False
return super().__eq__(actual)
def _yield_comparisons(self, actual):
return zip(actual, self.expected)
def _check_type(self) -> None:
__tracebackhide__ = True
for index, x in enumerate(self.expected):
if isinstance(x, type(self.expected)):
msg = "pytest.approx() does not support nested data structures: {!r} at index {}\n full sequence: {}"
raise TypeError(msg.format(x, index, pprint.pformat(self.expected)))
class ApproxScalar(ApproxBase):
"""Perform approximate comparisons where the expected value is a single number."""
# Using Real should be better than this Union, but not possible yet:
# https://github.com/python/typeshed/pull/3108
DEFAULT_ABSOLUTE_TOLERANCE: Union[float, Decimal] = 1e-12
DEFAULT_RELATIVE_TOLERANCE: Union[float, Decimal] = 1e-6
def __repr__(self) -> str:
"""Return a string communicating both the expected value and the
tolerance for the comparison being made.
For example, ``1.0 ± 1e-6``, ``(3+4j) ± 5e-6 ∠ ±180°``.
"""
# Don't show a tolerance for values that aren't compared using
# tolerances, i.e. non-numerics and infinities. Need to call abs to
# handle complex numbers, e.g. (inf + 1j).
if (not isinstance(self.expected, (Complex, Decimal))) or math.isinf(
abs(self.expected) # type: ignore[arg-type]
):
return str(self.expected)
# If a sensible tolerance can't be calculated, self.tolerance will
# raise a ValueError. In this case, display '???'.
try:
vetted_tolerance = f"{self.tolerance:.1e}"
if (
isinstance(self.expected, Complex)
and self.expected.imag
and not math.isinf(self.tolerance)
):
vetted_tolerance += " ∠ ±180°"
except ValueError:
vetted_tolerance = "???"
return f"{self.expected} ± {vetted_tolerance}"
def __eq__(self, actual) -> bool:
"""Return whether the given value is equal to the expected value
within the pre-specified tolerance."""
asarray = _as_numpy_array(actual)
if asarray is not None:
# Call ``__eq__()`` manually to prevent infinite-recursion with
# numpy<1.13. See #3748.
return all(self.__eq__(a) for a in asarray.flat)
# Short-circuit exact equality.
if actual == self.expected:
return True
# If either type is non-numeric, fall back to strict equality.
# NB: we need Complex, rather than just Number, to ensure that __abs__,
# __sub__, and __float__ are defined.
if not (
isinstance(self.expected, (Complex, Decimal))
and isinstance(actual, (Complex, Decimal))
):
return False
# Allow the user to control whether NaNs are considered equal to each
# other or not. The abs() calls are for compatibility with complex
# numbers.
if math.isnan(abs(self.expected)): # type: ignore[arg-type]
return self.nan_ok and math.isnan(abs(actual)) # type: ignore[arg-type]
# Infinity shouldn't be approximately equal to anything but itself, but
# if there's a relative tolerance, it will be infinite and infinity
# will seem approximately equal to everything. The equal-to-itself
# case would have been short circuited above, so here we can just
# return false if the expected value is infinite. The abs() call is
# for compatibility with complex numbers.
if math.isinf(abs(self.expected)): # type: ignore[arg-type]
return False
# Return true if the two numbers are within the tolerance.
result: bool = abs(self.expected - actual) <= self.tolerance
return result
# Ignore type because of https://github.com/python/mypy/issues/4266.
__hash__ = None # type: ignore
@property
def tolerance(self):
"""Return the tolerance for the comparison.
This could be either an absolute tolerance or a relative tolerance,
depending on what the user specified or which would be larger.
"""
def set_default(x, default):
return x if x is not None else default
# Figure out what the absolute tolerance should be. ``self.abs`` is
# either None or a value specified by the user.
absolute_tolerance = set_default(self.abs, self.DEFAULT_ABSOLUTE_TOLERANCE)
if absolute_tolerance < 0:
raise ValueError(
f"absolute tolerance can't be negative: {absolute_tolerance}"
)
if math.isnan(absolute_tolerance):
raise ValueError("absolute tolerance can't be NaN.")
# If the user specified an absolute tolerance but not a relative one,
# just return the absolute tolerance.
if self.rel is None:
if self.abs is not None:
return absolute_tolerance
# Figure out what the relative tolerance should be. ``self.rel`` is
# either None or a value specified by the user. This is done after
# we've made sure the user didn't ask for an absolute tolerance only,
# because we don't want to raise errors about the relative tolerance if
# we aren't even going to use it.
relative_tolerance = set_default(
self.rel, self.DEFAULT_RELATIVE_TOLERANCE
) * abs(self.expected)
if relative_tolerance < 0:
raise ValueError(
f"relative tolerance can't be negative: {relative_tolerance}"
)
if math.isnan(relative_tolerance):
raise ValueError("relative tolerance can't be NaN.")
# Return the larger of the relative and absolute tolerances.
return max(relative_tolerance, absolute_tolerance)
class ApproxDecimal(ApproxScalar):
"""Perform approximate comparisons where the expected value is a Decimal."""
DEFAULT_ABSOLUTE_TOLERANCE = Decimal("1e-12")
DEFAULT_RELATIVE_TOLERANCE = Decimal("1e-6")
[docs]def approx(expected, rel=None, abs=None, nan_ok: bool = False) -> ApproxBase:
"""Assert that two numbers (or two sets of numbers) are equal to each other
within some tolerance.
Due to the :std:doc:`tutorial/floatingpoint`, numbers that we
would intuitively expect to be equal are not always so::
>>> 0.1 + 0.2 == 0.3
False
This problem is commonly encountered when writing tests, e.g. when making
sure that floating-point values are what you expect them to be. One way to
deal with this problem is to assert that two floating-point numbers are
equal to within some appropriate tolerance::
>>> abs((0.1 + 0.2) - 0.3) < 1e-6
True
However, comparisons like this are tedious to write and difficult to
understand. Furthermore, absolute comparisons like the one above are
usually discouraged because there's no tolerance that works well for all
situations. ``1e-6`` is good for numbers around ``1``, but too small for
very big numbers and too big for very small ones. It's better to express
the tolerance as a fraction of the expected value, but relative comparisons
like that are even more difficult to write correctly and concisely.
The ``approx`` class performs floating-point comparisons using a syntax
that's as intuitive as possible::
>>> from pytest import approx
>>> 0.1 + 0.2 == approx(0.3)
True
The same syntax also works for sequences of numbers::
>>> (0.1 + 0.2, 0.2 + 0.4) == approx((0.3, 0.6))
True
Dictionary *values*::
>>> {'a': 0.1 + 0.2, 'b': 0.2 + 0.4} == approx({'a': 0.3, 'b': 0.6})
True
``numpy`` arrays::
>>> import numpy as np # doctest: +SKIP
>>> np.array([0.1, 0.2]) + np.array([0.2, 0.4]) == approx(np.array([0.3, 0.6])) # doctest: +SKIP
True
And for a ``numpy`` array against a scalar::
>>> import numpy as np # doctest: +SKIP
>>> np.array([0.1, 0.2]) + np.array([0.2, 0.1]) == approx(0.3) # doctest: +SKIP
True
By default, ``approx`` considers numbers within a relative tolerance of
``1e-6`` (i.e. one part in a million) of its expected value to be equal.
This treatment would lead to surprising results if the expected value was
``0.0``, because nothing but ``0.0`` itself is relatively close to ``0.0``.
To handle this case less surprisingly, ``approx`` also considers numbers
within an absolute tolerance of ``1e-12`` of its expected value to be
equal. Infinity and NaN are special cases. Infinity is only considered
equal to itself, regardless of the relative tolerance. NaN is not
considered equal to anything by default, but you can make it be equal to
itself by setting the ``nan_ok`` argument to True. (This is meant to
facilitate comparing arrays that use NaN to mean "no data".)
Both the relative and absolute tolerances can be changed by passing
arguments to the ``approx`` constructor::
>>> 1.0001 == approx(1)
False
>>> 1.0001 == approx(1, rel=1e-3)
True
>>> 1.0001 == approx(1, abs=1e-3)
True
If you specify ``abs`` but not ``rel``, the comparison will not consider
the relative tolerance at all. In other words, two numbers that are within
the default relative tolerance of ``1e-6`` will still be considered unequal
if they exceed the specified absolute tolerance. If you specify both
``abs`` and ``rel``, the numbers will be considered equal if either
tolerance is met::
>>> 1 + 1e-8 == approx(1)
True
>>> 1 + 1e-8 == approx(1, abs=1e-12)
False
>>> 1 + 1e-8 == approx(1, rel=1e-6, abs=1e-12)
True
You can also use ``approx`` to compare nonnumeric types, or dicts and
sequences containing nonnumeric types, in which case it falls back to
strict equality. This can be useful for comparing dicts and sequences that
can contain optional values::
>>> {"required": 1.0000005, "optional": None} == approx({"required": 1, "optional": None})
True
>>> [None, 1.0000005] == approx([None,1])
True
>>> ["foo", 1.0000005] == approx([None,1])
False
If you're thinking about using ``approx``, then you might want to know how
it compares to other good ways of comparing floating-point numbers. All of
these algorithms are based on relative and absolute tolerances and should
agree for the most part, but they do have meaningful differences:
- ``math.isclose(a, b, rel_tol=1e-9, abs_tol=0.0)``: True if the relative
tolerance is met w.r.t. either ``a`` or ``b`` or if the absolute
tolerance is met. Because the relative tolerance is calculated w.r.t.
both ``a`` and ``b``, this test is symmetric (i.e. neither ``a`` nor
``b`` is a "reference value"). You have to specify an absolute tolerance
if you want to compare to ``0.0`` because there is no tolerance by
default. More information: :py:func:`math.isclose`.
- ``numpy.isclose(a, b, rtol=1e-5, atol=1e-8)``: True if the difference
between ``a`` and ``b`` is less that the sum of the relative tolerance
w.r.t. ``b`` and the absolute tolerance. Because the relative tolerance
is only calculated w.r.t. ``b``, this test is asymmetric and you can
think of ``b`` as the reference value. Support for comparing sequences
is provided by :py:func:`numpy.allclose`. More information:
:std:doc:`numpy:reference/generated/numpy.isclose`.
- ``unittest.TestCase.assertAlmostEqual(a, b)``: True if ``a`` and ``b``
are within an absolute tolerance of ``1e-7``. No relative tolerance is
considered , so this function is not appropriate for very large or very
small numbers. Also, it's only available in subclasses of ``unittest.TestCase``
and it's ugly because it doesn't follow PEP8. More information:
:py:meth:`unittest.TestCase.assertAlmostEqual`.
- ``a == pytest.approx(b, rel=1e-6, abs=1e-12)``: True if the relative
tolerance is met w.r.t. ``b`` or if the absolute tolerance is met.
Because the relative tolerance is only calculated w.r.t. ``b``, this test
is asymmetric and you can think of ``b`` as the reference value. In the
special case that you explicitly specify an absolute tolerance but not a
relative tolerance, only the absolute tolerance is considered.
.. note::
``approx`` can handle numpy arrays, but we recommend the
specialised test helpers in :std:doc:`numpy:reference/routines.testing`
if you need support for comparisons, NaNs, or ULP-based tolerances.
.. warning::
.. versionchanged:: 3.2
In order to avoid inconsistent behavior, :py:exc:`TypeError` is
raised for ``>``, ``>=``, ``<`` and ``<=`` comparisons.
The example below illustrates the problem::
assert approx(0.1) > 0.1 + 1e-10 # calls approx(0.1).__gt__(0.1 + 1e-10)
assert 0.1 + 1e-10 > approx(0.1) # calls approx(0.1).__lt__(0.1 + 1e-10)
In the second example one expects ``approx(0.1).__le__(0.1 + 1e-10)``
to be called. But instead, ``approx(0.1).__lt__(0.1 + 1e-10)`` is used to
comparison. This is because the call hierarchy of rich comparisons
follows a fixed behavior. More information: :py:meth:`object.__ge__`
.. versionchanged:: 3.7.1
``approx`` raises ``TypeError`` when it encounters a dict value or
sequence element of nonnumeric type.
.. versionchanged:: 6.1.0
``approx`` falls back to strict equality for nonnumeric types instead
of raising ``TypeError``.
"""
# Delegate the comparison to a class that knows how to deal with the type
# of the expected value (e.g. int, float, list, dict, numpy.array, etc).
#
# The primary responsibility of these classes is to implement ``__eq__()``
# and ``__repr__()``. The former is used to actually check if some
# "actual" value is equivalent to the given expected value within the
# allowed tolerance. The latter is used to show the user the expected
# value and tolerance, in the case that a test failed.
#
# The actual logic for making approximate comparisons can be found in
# ApproxScalar, which is used to compare individual numbers. All of the
# other Approx classes eventually delegate to this class. The ApproxBase
# class provides some convenient methods and overloads, but isn't really
# essential.
__tracebackhide__ = True
if isinstance(expected, Decimal):
cls: Type[ApproxBase] = ApproxDecimal
elif isinstance(expected, Mapping):
cls = ApproxMapping
elif _is_numpy_array(expected):
expected = _as_numpy_array(expected)
cls = ApproxNumpy
elif (
isinstance(expected, Iterable)
and isinstance(expected, Sized)
# Type ignored because the error is wrong -- not unreachable.
and not isinstance(expected, STRING_TYPES) # type: ignore[unreachable]
):
cls = ApproxSequencelike
else:
cls = ApproxScalar
return cls(expected, rel, abs, nan_ok)
def _is_numpy_array(obj: object) -> bool:
"""
Return true if the given object is implicitly convertible to ndarray,
and numpy is already imported.
"""
return _as_numpy_array(obj) is not None
def _as_numpy_array(obj: object) -> Optional["ndarray"]:
"""
Return an ndarray if the given object is implicitly convertible to ndarray,
and numpy is already imported, otherwise None.
"""
import sys
np: Any = sys.modules.get("numpy")
if np is not None:
# avoid infinite recursion on numpy scalars, which have __array__
if np.isscalar(obj):
return None
elif isinstance(obj, np.ndarray):
return obj
elif hasattr(obj, "__array__") or hasattr("obj", "__array_interface__"):
return np.asarray(obj)
return None
# builtin pytest.raises helper
E = TypeVar("E", bound=BaseException)
@overload
def raises(
expected_exception: Union[Type[E], Tuple[Type[E], ...]],
*,
match: Optional[Union[str, Pattern[str]]] = ...,
) -> "RaisesContext[E]":
...
@overload
def raises(
expected_exception: Union[Type[E], Tuple[Type[E], ...]],
func: Callable[..., Any],
*args: Any,
**kwargs: Any,
) -> _pytest._code.ExceptionInfo[E]:
...
[docs]def raises(
expected_exception: Union[Type[E], Tuple[Type[E], ...]], *args: Any, **kwargs: Any
) -> Union["RaisesContext[E]", _pytest._code.ExceptionInfo[E]]:
r"""Assert that a code block/function call raises ``expected_exception``
or raise a failure exception otherwise.
:kwparam match:
If specified, a string containing a regular expression,
or a regular expression object, that is tested against the string
representation of the exception using :py:func:`re.search`. To match a literal
string that may contain :std:ref:`special characters <re-syntax>`, the pattern can
first be escaped with :py:func:`re.escape`.
(This is only used when :py:func:`pytest.raises` is used as a context manager,
and passed through to the function otherwise.
When using :py:func:`pytest.raises` as a function, you can use:
``pytest.raises(Exc, func, match="passed on").match("my pattern")``.)
.. currentmodule:: _pytest._code
Use ``pytest.raises`` as a context manager, which will capture the exception of the given
type::
>>> import pytest
>>> with pytest.raises(ZeroDivisionError):
... 1/0
If the code block does not raise the expected exception (``ZeroDivisionError`` in the example
above), or no exception at all, the check will fail instead.
You can also use the keyword argument ``match`` to assert that the
exception matches a text or regex::
>>> with pytest.raises(ValueError, match='must be 0 or None'):
... raise ValueError("value must be 0 or None")
>>> with pytest.raises(ValueError, match=r'must be \d+$'):
... raise ValueError("value must be 42")
The context manager produces an :class:`ExceptionInfo` object which can be used to inspect the
details of the captured exception::
>>> with pytest.raises(ValueError) as exc_info:
... raise ValueError("value must be 42")
>>> assert exc_info.type is ValueError
>>> assert exc_info.value.args[0] == "value must be 42"
.. note::
When using ``pytest.raises`` as a context manager, it's worthwhile to
note that normal context manager rules apply and that the exception
raised *must* be the final line in the scope of the context manager.
Lines of code after that, within the scope of the context manager will
not be executed. For example::
>>> value = 15
>>> with pytest.raises(ValueError) as exc_info:
... if value > 10:
... raise ValueError("value must be <= 10")
... assert exc_info.type is ValueError # this will not execute
Instead, the following approach must be taken (note the difference in
scope)::
>>> with pytest.raises(ValueError) as exc_info:
... if value > 10:
... raise ValueError("value must be <= 10")
...
>>> assert exc_info.type is ValueError
**Using with** ``pytest.mark.parametrize``
When using :ref:`pytest.mark.parametrize ref`
it is possible to parametrize tests such that
some runs raise an exception and others do not.
See :ref:`parametrizing_conditional_raising` for an example.
**Legacy form**
It is possible to specify a callable by passing a to-be-called lambda::
>>> raises(ZeroDivisionError, lambda: 1/0)
<ExceptionInfo ...>
or you can specify an arbitrary callable with arguments::
>>> def f(x): return 1/x
...
>>> raises(ZeroDivisionError, f, 0)
<ExceptionInfo ...>
>>> raises(ZeroDivisionError, f, x=0)
<ExceptionInfo ...>
The form above is fully supported but discouraged for new code because the
context manager form is regarded as more readable and less error-prone.
.. note::
Similar to caught exception objects in Python, explicitly clearing
local references to returned ``ExceptionInfo`` objects can
help the Python interpreter speed up its garbage collection.
Clearing those references breaks a reference cycle
(``ExceptionInfo`` --> caught exception --> frame stack raising
the exception --> current frame stack --> local variables -->
``ExceptionInfo``) which makes Python keep all objects referenced
from that cycle (including all local variables in the current
frame) alive until the next cyclic garbage collection run.
More detailed information can be found in the official Python
documentation for :ref:`the try statement <python:try>`.
"""
__tracebackhide__ = True
if isinstance(expected_exception, type):
excepted_exceptions: Tuple[Type[E], ...] = (expected_exception,)
else:
excepted_exceptions = expected_exception
for exc in excepted_exceptions:
if not isinstance(exc, type) or not issubclass(exc, BaseException):
msg = "expected exception must be a BaseException type, not {}" # type: ignore[unreachable]
not_a = exc.__name__ if isinstance(exc, type) else type(exc).__name__
raise TypeError(msg.format(not_a))
message = f"DID NOT RAISE {expected_exception}"
if not args:
match: Optional[Union[str, Pattern[str]]] = kwargs.pop("match", None)
if kwargs:
msg = "Unexpected keyword arguments passed to pytest.raises: "
msg += ", ".join(sorted(kwargs))
msg += "\nUse context-manager form instead?"
raise TypeError(msg)
return RaisesContext(expected_exception, message, match)
else:
func = args[0]
if not callable(func):
raise TypeError(f"{func!r} object (type: {type(func)}) must be callable")
try:
func(*args[1:], **kwargs)
except expected_exception as e:
# We just caught the exception - there is a traceback.
assert e.__traceback__ is not None
return _pytest._code.ExceptionInfo.from_exc_info(
(type(e), e, e.__traceback__)
)
fail(message)
# This doesn't work with mypy for now. Use fail.Exception instead.
raises.Exception = fail.Exception # type: ignore
@final
class RaisesContext(Generic[E]):
def __init__(
self,
expected_exception: Union[Type[E], Tuple[Type[E], ...]],
message: str,
match_expr: Optional[Union[str, Pattern[str]]] = None,
) -> None:
self.expected_exception = expected_exception
self.message = message
self.match_expr = match_expr
self.excinfo: Optional[_pytest._code.ExceptionInfo[E]] = None
def __enter__(self) -> _pytest._code.ExceptionInfo[E]:
self.excinfo = _pytest._code.ExceptionInfo.for_later()
return self.excinfo
def __exit__(
self,
exc_type: Optional[Type[BaseException]],
exc_val: Optional[BaseException],
exc_tb: Optional[TracebackType],
) -> bool:
__tracebackhide__ = True
if exc_type is None:
fail(self.message)
assert self.excinfo is not None
if not issubclass(exc_type, self.expected_exception):
return False
# Cast to narrow the exception type now that it's verified.
exc_info = cast(Tuple[Type[E], E, TracebackType], (exc_type, exc_val, exc_tb))
self.excinfo.fill_unfilled(exc_info)
if self.match_expr is not None:
self.excinfo.match(self.match_expr)
return True
```