python中的eval函数的用法_isnan函数

a=10;
print(eval("a+1"))

a=10;
g={ 
   'a':4}
print(eval("a+1",g))

a=10
b=20
c=30
g={ 
   'a':6,'b':8}
t={ 
   'b':100,'c':10}
print(eval('a+b+c',g,t))

   The arguments are a string and optional globals and locals. If provided, globals must be a 
dictionary. If provided, locals can be any mapping object.
The expression argument is parsed and evaluated as a Python expression (technically speaking, 
a condition list) using the globals and locals dictionaries as global and local namespace. If the
globals dictionary is present and lacks ‘__builtins__’, the current globals are copied into 
globals before expression is parsed. This means that expression normally has full access to the 
standard builtins module and restricted environments are propagated. If the locals dictionary is 
omitted it defaults to the globals dictionary. If both dictionaries are omitted, the expression 
is executed in the environment where eval() is called. The return value is the result of the 
evaluated expression. Syntax errors are reported as exceptions. Example:
>>> x = 1
>>> eval('x+1')
2
This function can also be used to execute arbitrary code objects (such as those created by 
compile()). In this case pass a code object instead of a string. If the code object has been
compiled with 'exec' as the mode argument, eval()‘s return value will be None.
Hints: dynamic execution of statements is supported by the exec() function. The globals() and 
locals() functions returns the current global and local dictionary, respectively, which may be 
useful to pass around for use by eval() or exec().
See ast.literal_eval() for a function that can safely evaluate strings with expressions 
containing only literals.