#!/usr/bin/env python
"""Top level ``eval`` module.
"""
import tokenize
import warnings
from pandas.compat import string_types
from pandas.util._validators import validate_bool_kwarg
from pandas.core.computation.engines import _engines
from pandas.core.computation.scope import _ensure_scope
from pandas.io.formats.printing import pprint_thing
def _check_engine(engine):
"""Make sure a valid engine is passed.
Parameters
----------
engine : str
Raises
------
KeyError
* If an invalid engine is passed
ImportError
* If numexpr was requested but doesn't exist
Returns
-------
string engine
"""
from pandas.core.computation.check import _NUMEXPR_INSTALLED
if engine is None:
if _NUMEXPR_INSTALLED:
engine = 'numexpr'
else:
engine = 'python'
if engine not in _engines:
valid = list(_engines.keys())
raise KeyError('Invalid engine {engine!r} passed, valid engines are'
' {valid}'.format(engine=engine, valid=valid))
# TODO: validate this in a more general way (thinking of future engines
# that won't necessarily be import-able)
# Could potentially be done on engine instantiation
if engine == 'numexpr':
if not _NUMEXPR_INSTALLED:
raise ImportError("'numexpr' is not installed or an "
"unsupported version. Cannot use "
"engine='numexpr' for query/eval "
"if 'numexpr' is not installed")
return engine
def _check_parser(parser):
"""Make sure a valid parser is passed.
Parameters
----------
parser : str
Raises
------
KeyError
* If an invalid parser is passed
"""
from pandas.core.computation.expr import _parsers
if parser not in _parsers:
raise KeyError('Invalid parser {parser!r} passed, valid parsers are'
' {valid}'.format(parser=parser, valid=_parsers.keys()))
def _check_resolvers(resolvers):
if resolvers is not None:
for resolver in resolvers:
if not hasattr(resolver, '__getitem__'):
name = type(resolver).__name__
raise TypeError('Resolver of type {name!r} does not implement '
'the __getitem__ method'.format(name=name))
def _check_expression(expr):
"""Make sure an expression is not an empty string
Parameters
----------
expr : object
An object that can be converted to a string
Raises
------
ValueError
* If expr is an empty string
"""
if not expr:
raise ValueError("expr cannot be an empty string")
def _convert_expression(expr):
"""Convert an object to an expression.
Thus function converts an object to an expression (a unicode string) and
checks to make sure it isn't empty after conversion. This is used to
convert operators to their string representation for recursive calls to
:func:`~pandas.eval`.
Parameters
----------
expr : object
The object to be converted to a string.
Returns
-------
s : unicode
The string representation of an object.
Raises
------
ValueError
* If the expression is empty.
"""
s = pprint_thing(expr)
_check_expression(s)
return s
def _check_for_locals(expr, stack_level, parser):
from pandas.core.computation.expr import tokenize_string
at_top_of_stack = stack_level == 0
not_pandas_parser = parser != 'pandas'
if not_pandas_parser:
msg = "The '@' prefix is only supported by the pandas parser"
elif at_top_of_stack:
msg = ("The '@' prefix is not allowed in "
"top-level eval calls, \nplease refer to "
"your variables by name without the '@' "
"prefix")
if at_top_of_stack or not_pandas_parser:
for toknum, tokval in tokenize_string(expr):
if toknum == tokenize.OP and tokval == '@':
raise SyntaxError(msg)
def eval(expr, parser='pandas', engine=None, truediv=True,
local_dict=None, global_dict=None, resolvers=(), level=0,
target=None, inplace=False):
"""Evaluate a Python expression as a string using various backends.
The following arithmetic operations are supported: ``+``, ``-``, ``*``,
``/``, ``**``, ``%``, ``//`` (python engine only) along with the following
boolean operations: ``|`` (or), ``&`` (and), and ``~`` (not).
Additionally, the ``'pandas'`` parser allows the use of :keyword:`and`,
:keyword:`or`, and :keyword:`not` with the same semantics as the
corresponding bitwise operators. :class:`~pandas.Series` and
:class:`~pandas.DataFrame` objects are supported and behave as they would
with plain ol' Python evaluation.
Parameters
----------
expr : str or unicode
The expression to evaluate. This string cannot contain any Python
`statements
<https://docs.python.org/3/reference/simple_stmts.html#simple-statements>`__,
only Python `expressions
<https://docs.python.org/3/reference/simple_stmts.html#expression-statements>`__.
parser : string, default 'pandas', {'pandas', 'python'}
The parser to use to construct the syntax tree from the expression. The
default of ``'pandas'`` parses code slightly different than standard
Python. Alternatively, you can parse an expression using the
``'python'`` parser to retain strict Python semantics. See the
:ref:`enhancing performance <enhancingperf.eval>` documentation for
more details.
engine : string or None, default 'numexpr', {'python', 'numexpr'}
The engine used to evaluate the expression. Supported engines are
- None : tries to use ``numexpr``, falls back to ``python``
- ``'numexpr'``: This default engine evaluates pandas objects using
numexpr for large speed ups in complex expressions
with large frames.
- ``'python'``: Performs operations as if you had ``eval``'d in top
level python. This engine is generally not that useful.
More backends may be available in the future.
truediv : bool, optional
Whether to use true division, like in Python >= 3
local_dict : dict or None, optional
A dictionary of local variables, taken from locals() by default.
global_dict : dict or None, optional
A dictionary of global variables, taken from globals() by default.
resolvers : list of dict-like or None, optional
A list of objects implementing the ``__getitem__`` special method that
you can use to inject an additional collection of namespaces to use for
variable lookup. For example, this is used in the
:meth:`~pandas.DataFrame.query` method to inject the
``DataFrame.index`` and ``DataFrame.columns``
variables that refer to their respective :class:`~pandas.DataFrame`
instance attributes.
level : int, optional
The number of prior stack frames to traverse and add to the current
scope. Most users will **not** need to change this parameter.
target : object, optional, default None
This is the target object for assignment. It is used when there is
variable assignment in the expression. If so, then `target` must
support item assignment with string keys, and if a copy is being
returned, it must also support `.copy()`.
inplace : bool, default False
If `target` is provided, and the expression mutates `target`, whether
to modify `target` inplace. Otherwise, return a copy of `target` with
the mutation.
Returns
-------
ndarray, numeric scalar, DataFrame, Series
Raises
------
ValueError
There are many instances where such an error can be raised:
- `target=None`, but the expression is multiline.
- The expression is multiline, but not all them have item assignment.
An example of such an arrangement is this:
a = b + 1
a + 2
Here, there are expressions on different lines, making it multiline,
but the last line has no variable assigned to the output of `a + 2`.
- `inplace=True`, but the expression is missing item assignment.
- Item assignment is provided, but the `target` does not support
string item assignment.
- Item assignment is provided and `inplace=False`, but the `target`
does not support the `.copy()` method
See Also
--------
pandas.DataFrame.query
pandas.DataFrame.eval
Notes
-----
The ``dtype`` of any objects involved in an arithmetic ``%`` operation are
recursively cast to ``float64``.
See the :ref:`enhancing performance <enhancingperf.eval>` documentation for
more details.
"""
from pandas.core.computation.expr import Expr
inplace = validate_bool_kwarg(inplace, "inplace")
if isinstance(expr, string_types):
_check_expression(expr)
exprs = [e.strip() for e in expr.splitlines() if e.strip() != '']
else:
exprs = [expr]
multi_line = len(exprs) > 1
if multi_line and target is None:
raise ValueError("multi-line expressions are only valid in the "
"context of data, use DataFrame.eval")
ret = None
first_expr = True
target_modified = False
for expr in exprs:
expr = _convert_expression(expr)
engine = _check_engine(engine)
_check_parser(parser)
_check_resolvers(resolvers)
_check_for_locals(expr, level, parser)
# get our (possibly passed-in) scope
env = _ensure_scope(level + 1, global_dict=global_dict,
local_dict=local_dict, resolvers=resolvers,
target=target)
parsed_expr = Expr(expr, engine=engine, parser=parser, env=env,
truediv=truediv)
# construct the engine and evaluate the parsed expression
eng = _engines[engine]
eng_inst = eng(parsed_expr)
ret = eng_inst.evaluate()
if parsed_expr.assigner is None:
if multi_line:
raise ValueError("Multi-line expressions are only valid"
" if all expressions contain an assignment")
elif inplace:
raise ValueError("Cannot operate inplace "
"if there is no assignment")
# assign if needed
assigner = parsed_expr.assigner
if env.target is not None and assigner is not None:
target_modified = True
# if returning a copy, copy only on the first assignment
if not inplace and first_expr:
try:
target = env.target.copy()
except AttributeError:
raise ValueError("Cannot return a copy of the target")
else:
target = env.target
# TypeError is most commonly raised (e.g. int, list), but you
# get IndexError if you try to do this assignment on np.ndarray.
# we will ignore numpy warnings here; e.g. if trying
# to use a non-numeric indexer
try:
with warnings.catch_warnings(record=True):
# TODO: Filter the warnings we actually care about here.
target[assigner] = ret
except (TypeError, IndexError):
raise ValueError("Cannot assign expression output to target")
if not resolvers:
resolvers = ({assigner: ret},)
else:
# existing resolver needs updated to handle
# case of mutating existing column in copy
for resolver in resolvers:
if assigner in resolver:
resolver[assigner] = ret
break
else:
resolvers += ({assigner: ret},)
ret = None
first_expr = False
# We want to exclude `inplace=None` as being False.
if inplace is False:
return target if target_modified else ret
