problem in output of a function

[akbarza]

hi
the below code is in https://realpython.com/python-eval-function/ :

def eval_expression(input_string):
    # Step 1
    allowed_names = {"sum": sum}
    # Step 2
    code = compile(input_string, "<string>", "eval")
    # Step 3
    for name in code.co_names:
        if name not in allowed_names:
            # Step 4
            raise NameError(f"Use of {name} not allowed")
    return eval(code, {"__builtins__": {}}, allowed_names)

eval_expression("pow(10, 2)")

when I run it in idle, my output is :

Error:
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 10, in eval_expression
NameError: Use of pow not allowed
    raise NameError(f"Use of {name} not allowed")
NameError: Use of pow not allowed

in above output, in the 5'th line the{name} must be the pow as it is in the output of the site.
what is the problem?
thanks

[deanhystad]

If you want to evaluate an expression that contains "pow", you need to add "pow" to allowed_names.

[akbarza]

If you want to evaluate an expression that contains "pow", you need to add "pow" to allowed_names.

in error message, in the line before last line, {name} must be pow, but as you can see it is {name}. why?

[deanhystad]

This is the python expression that raises the exception.

raise NameError(f"Use of {name} not allowed")

It is line 10 in your posted code.
The expression on line 10 raises an exception with this message:

Error:
NameError: Use of pow not allowed

The f"string replaces {name} with "pow", the str referenced by name. This changes the str passed to the exception. It does not rewrite the code on line 10.

Another example might help. I run this code:

x = list("ABC")
for i in range(4):
    print(x[i])

I get an index error.

Error:
A
B
C
Traceback (most recent call last):
  File "...", line 3, in <module>
    print(x[i])
IndexError: list index out of range

The line before the "IndexError" message is the statement that raised the exception. It says "print(x[i])". It does not say "print(x[3])". Granted, it would be very cool if error reporting replaced variables with their current values at the time of the error, but that is not what Python does. It might not even be possible. Accessing the variable value may be the reason for the exception.

[akbarza]

This is the python expression that raises the exception.

raise NameError(f"Use of {name} not allowed")

It is line 10 in your posted code.
The expression on line 10 raises an exception with this message:

Error:
NameError: Use of pow not allowed

The f"string replaces {name} with "pow", the str referenced by name. This changes the str passed to the exception. It does not rewrite the code on line 10. Understand?

yes, i understand

[deanhystad]

You are too quick. Or I am too slow. I really like the second example I thought of after my initial reply. Much better than "Understand?"

[akbarza]

You are too quick. Or I am too slow. I really like the second example I thought of after my initial reply. Much better than "Understand?"

hi, and thanks for the reply
can I ask about

code.co_names

? what does co_names do? where can I get more info about it?
On the whole, when I encounter a new word or thing in a code, how can I get info about it(other than googling)?
thanks again

[snippsat]

? what does co_names do? where can I get more info about it?

It's an attribute(method) of built-in compile().
So if want look more at it has to look tutorials about compile(),
and not .co_names alone which is just one method.

>>> code = compile('print(55)', 'test', 'eval')
>>> exec(code)
55

# Use a couple of methods 
>>> code.co_names
('print',)
>>> code.co_code
b'\x97\x00\x02\x00e\x00d\x00\xa6\x01\x00\x00\xab\x01\x00\x00\x00\x00\x00\x00\x00\x00S\x00'

# All 
>>> [i for i in dir(code) if not i.startswith('__')]
['_co_code_adaptive', '_varname_from_oparg', 'co_argcount', 'co_cellvars', 'co_code',
'co_consts', 'co_exceptiontable', 'co_filename', 'co_firstlineno', 'co_flags', 'co_freevars',
'co_kwonlyargcount', 'co_lines', 'co_linetable', 'co_lnotab', 'co_name', 'co_names', 'co_nlocals',
'co_positions', 'co_posonlyargcount', 'co_qualname', 'co_stacksize', 'co_varnames', 'replace']

# look at help for compile
>>> help(compile)
Help on built-in function compile in module builtins:

compile(source, filename, mode, flags=0, dont_inherit=False, optimize=-1, *, _feature_version=-1)
    Compile source into a code object that can be executed by exec() or eval().

    The source code may represent a Python module, statement or expression.
    The filename will be used for run-time error messages.
    The mode must be 'exec' to compile a module, 'single' to compile a
    single (interactive) statement, or 'eval' to compile an expression.
    The flags argument, if present, controls which future statements influence
    the compilation of the code.
    The dont_inherit argument, if true, stops the compilation inheriting
    the effects of any future statements in effect in the code calling
    compile; if absent or false these statements do influence the compilation,
    in addition to any features explicitly specified.

[akbarza]

? what does co_names do? where can I get more info about it?

It's an attribute(method) of built-in compile().
So if want look more at it has to look tutorials about compile(),
and not .co_names alone which is just one method.

>>> code = compile('print(55)', 'test', 'eval')
>>> exec(code)
55

# Use a couple of methods 
>>> code.co_names
('print',)
>>> code.co_code
b'\x97\x00\x02\x00e\x00d\x00\xa6\x01\x00\x00\xab\x01\x00\x00\x00\x00\x00\x00\x00\x00S\x00'

# All 
>>> [i for i in dir(code) if not i.startswith('__')]
['_co_code_adaptive', '_varname_from_oparg', 'co_argcount', 'co_cellvars', 'co_code',
'co_consts', 'co_exceptiontable', 'co_filename', 'co_firstlineno', 'co_flags', 'co_freevars',
'co_kwonlyargcount', 'co_lines', 'co_linetable', 'co_lnotab', 'co_name', 'co_names', 'co_nlocals',
'co_positions', 'co_posonlyargcount', 'co_qualname', 'co_stacksize', 'co_varnames', 'replace']

# look at help for compile
>>> help(compile)
Help on built-in function compile in module builtins:

compile(source, filename, mode, flags=0, dont_inherit=False, optimize=-1, *, _feature_version=-1)
    Compile source into a code object that can be executed by exec() or eval().

    The source code may represent a Python module, statement or expression.
    The filename will be used for run-time error messages.
    The mode must be 'exec' to compile a module, 'single' to compile a
    single (interactive) statement, or 'eval' to compile an expression.
    The flags argument, if present, controls which future statements influence
    the compilation of the code.
    The dont_inherit argument, if true, stops the compilation inheriting
    the effects of any future statements in effect in the code calling
    compile; if absent or false these statements do influence the compilation,
    in addition to any features explicitly specified.

hi and thanks for replying
if I want to know about co_code or co_names, how can I do it?
i wrote

help(compile.co_code)

and

compile.co_names.__doc__ 

, but i get error.

[snippsat]

if I want to know about co_code or co_names, how can I do it?

They are method of complie(),so you most use compile first.

>>> code = compile('print(55)', 'test', 'eval')
>>> help(code.co_names)
Help on tuple object:

class tuple(object)
 |  tuple(iterable=(), /)
.....

There is no doc string so help will only return help on tuple return.
To see some help on this can look at Code objects,so this a more special case than most other stuff.
If eg use Rich can see all methods at once.

>>>from rich import inspect

>>> code = compile('print(55)', 'test', 'eval')
>>> inspect(code)
┌────────────────────────────── <class 'code'> ───────────────────────────────┐
│ Create a code object.  Not for the faint of heart.                          │
│                                                                             │
│ ┌─────────────────────────────────────────────────────────────────────────┐ │
│ │ <code object <module> at 0x0000015DAB3A86B0, file "test", line 1>       │ │
│ └─────────────────────────────────────────────────────────────────────────┘ │
│                                                                             │
│        co_argcount = 0                                                      │
│        co_cellvars = ()                                                     │
│            co_code = b'\x97\x00\x02\x00e\x00d\x00\xa6\x01\x00\x00\xab\x01\… │
│          co_consts = (55,)                                                  │
│  co_exceptiontable = b''                                                    │
│        co_filename = 'test'                                                 │
│     co_firstlineno = 1                                                      │
│           co_flags = 0                                                      │
│        co_freevars = ()                                                     │
│  co_kwonlyargcount = 0                                                      │
│       co_linetable = b'\xf0\x03\x01\x01\x01\xd8\x00\x05\x80\x05\x80b\x81\t… │
│          co_lnotab = b'\x00\xff\x02\x01'                                    │
│            co_name = '<module>'                                             │
│           co_names = ('print',)                                             │
│         co_nlocals = 0                                                      │
│ co_posonlyargcount = 0                                                      │
│        co_qualname = '<module>'                                             │
│       co_stacksize = 3                                                      │
│        co_varnames = ()                                                     │
└─────────────────────────────────────────────────────────────────────────────┘

---

Here how it look if use eg make string hello,then will see that it has doc string for all string methods.