Using Asteval¶
This chapter gives a quick overview of asteval, showing basic usage and the most important features. Further details can be found in the next chapter (Asteval Reference).
Creating and using an asteval Interpreter¶
The asteval module is very easy to use. Import the module and create an Interpreter:
>>> from asteval import Interpreter
>>> aeval = Interpreter()
and now you have an embedded interpreter for a procedural, mathematical language that is very much like python:
>>> aeval('x = sqrt(3)')
>>> aeval('print(x)')
1.73205080757
>>> aeval('''
for i in range(10):
print(i, sqrt(i), log(1+1))
''')
0 0.0 0.0
1 1.0 0.69314718056
2 1.41421356237 1.09861228867
3 1.73205080757 1.38629436112
4 2.0 1.60943791243
5 2.2360679775 1.79175946923
6 2.44948974278 1.94591014906
7 2.64575131106 2.07944154168
8 2.82842712475 2.19722457734
9 3.0 2.30258509299
There are lots of options when creating the Interpreter to controller what functionality is and isn’t allowed and to pre-load data and functions. The default interpreter gives a limited but useful version of the Python language.
accessing the symbol table¶
The symbol table (that is, the mapping between variable and function names and
the underlying objects) is a simple dictionary (by default, see
Symbol Tables used in asteval for details of an optional alternative) held in the
symtable
attribute of the interpreter, and can be read or written to:
>>> aeval('x = sqrt(3)')
>>> aeval.symtable['x']
1.73205080757
>>> aeval.symtable['y'] = 100
>>> aeval('print(y/8)')
12.5
Note here the use of true division even though the operands are integers.
As with Python itself, valid symbol names must match the basic regular expression pattern:
valid_name = [a-zA-Z_][a-zA-Z0-9_]*
In addition, certain names are reserved in Python, and cannot be used within the asteval interpreter. These reserved words are:
and, as, assert, async, await, break, class, continue, def, del, elif, else, eval, except, exec, execfile, finally, for, from, global, if, import, in, is, lambda, nonlocal, not, or, pass, print, raise, return, try, while, with, True, False, None, __import__, __package__
built-in functions¶
At startup, many symbols are loaded into the symbol table from Python’s
builtins and the math
module. The builtins include several basic Python
functions:
abs, all, any, bin, bool, bytearray, bytes, chr, complex, dict, dir, divmod, enumerate, filter, float, format, frozenset, hash, hex, id, int, isinstance, len, list, map, max, min, oct, ord, pow, range, repr, reversed, round, set, slice, sorted, str, sum, tuple, zip
and a large number of named exceptions:
ArithmeticError, AssertionError, AttributeError, BaseException, BufferError, BytesWarning, DeprecationWarning, EOFError, EnvironmentError, Exception, False, FloatingPointError, GeneratorExit, IOError, ImportError, ImportWarning, IndentationError, IndexError, KeyError, KeyboardInterrupt, LookupError, MemoryError, NameError, None, NotImplemented, NotImplementedError, OSError, OverflowError, ReferenceError, RuntimeError, RuntimeWarning, StopIteration, SyntaxError, SyntaxWarning, SystemError, SystemExit, True, TypeError, UnboundLocalError, UnicodeDecodeError, UnicodeEncodeError, UnicodeError, UnicodeTranslateError, UnicodeWarning, ValueError, Warning, ZeroDivisionError
The symbols imported from Python’s math
module include:
acos, acosh, asin, asinh, atan, atan2, atanh, ceil, copysign, cos, cosh, degrees, e, exp, fabs, factorial, floor, fmod, frexp, fsum, hypot, isinf, isnan, ldexp, log, log10, log1p, modf, pi, pow, radians, sin, sinh, sqrt, tan, tanh, trunc
If available, about 300 additional symbols are imported from numpy.
conditionals and loops¶
If-then-else blocks, for-loops (including the optional else
block),
while
loops (also including optional else
block), and with
blocks
are supported, and work exactly as they do in python. Thus:
>>> code = """
sum = 0
for i in range(10):
sum += i*sqrt(*1.0)
if i % 4 == 0:
sum = sum + 1
print("sum = ", sum)
"""
>>> aeval(code)
sum = 114.049534067
comprehensions¶
list, dict, and set comprehension are supported, acting just as they do in Python. Generators, yield, and async programming are not currently supported.
printing¶
For printing, asteval emulates Python’s native print()
function. You
can change where output is sent with the writer
argument when creating
the interpreter, or suppress printing all together with the no_print
option. By default, outputs are sent to sys.stdout
.
writing functions¶
User-defined functions can be written and executed, as in python with a
def
block, for example:
>>> from asteval import Interpreter
>>> aeval = Interpreter()
>>> code = """def func(a, b, norm=1.0):
... return (a + b)/norm
... """
>>> aeval(code)
>>> aeval("func(1, 3, norm=10.0)")
0.4
exceptions¶
Asteval monitors and caches exceptions in the evaluated code. Brief error
messages are printed (with Python’s print function, and so using standard
output by default), and the full set of exceptions is kept in the
error
attribute of the Interpreter
instance. This
error
attribute is a list of instances of the asteval
ExceptionHolder
class, which is accessed through the
get_error()
method. The error
attribute is reset to an empty
list at the beginning of each eval()
, so that errors are from only
the most recent eval()
.
Thus, to handle and re-raise exceptions from your Python code in a simple REPL loop, you’d want to do something similar to
>>> from asteval import Interpreter
>>> aeval = Interpreter()
>>> while True:
>>> inp_string = raw_input('dsl:>')
>>> result = aeval(inp_string)
>>> if len(aeval.error)>0:
>>> for err in aeval.error:
>>> print(err.get_error())
>>> else:
>>> print(result)