ValueError: malformed node or string

Question

Why do I get this error message: ValueError: malformed node or string when I pass data in the below format into the "parse_webhook" function?

Thanks!

webhook_data = {"side": "BUY","key": "8234023409fa3242309sdfasdf903024917325"}

def parse_webhook(webhook_data):

    """
    :param webhook_data: POST data from tradingview, as a string.
    :return: Dictionary version of string.
    """

    data = ast.literal_eval(webhook_data)
    return data

Wrror I get:

  File "C:\Users\User\anaconda3\lib\ast.py", line 55, in _convert_num
    raise ValueError('malformed node or string: ' + repr(node))

ValueError: malformed node or string: {'side': 'BUY', 'key': '8234023409fa3242309sdfasdf903024917325'}

Answer 1

ast.literal.eval:

Safely evaluate an expression node or a Unicode or Latin-1 encoded string containing a Python expression. The string or node provided may only consist of the following Python literal structures: strings, numbers, tuples, lists, dicts, booleans, and None.

You can't pass a dictionary to ast.literal_eval. It only takes a string input and converts it into appropriate python object. You can't pass lists / dictionary / tuples, etc to the ast.literal_eval .

---

Use Case:

The following snippet would have been a valid use of this method -

import ast
# Converting a string representation of dictionary into dictionary.
webhook_data = ast.literal_eval('{"side": "BUY","key": "8234023409fa3242309sdfasdf903024917325"}')
print(webhook_data)

Output:

{'side': 'BUY', 'key': '8234023409fa3242309sdfasdf903024917325'}

You could also use it when you have a list, tuple, booleans, numbers, sets etc in form of string and this method would evaluate it into appropriate python object.

---

Possible Fix to Error:

You already have a dictionary constructed with you. You could make your code work by doing the following edit:

webhook_data = {"side": "BUY","key": "8234023409fa3242309sdfasdf903024917325"}

def parse_webhook(webhook_data):

    """
    :param webhook_data: POST data from tradingview, as a string.
    :return: Dictionary version of string.
    """

    data = ast.literal_eval(str(webhook_data))
    return data

But the above code would just be redundant. You are just converting a dictionary to string using str() and that converted string back into dictionary using a ast.literal_eval.

---

NOTE:

1. It is possible to crash the Python interpreter with a sufficiently large/complex string due to stack depth limitations in Python’s AST compiler.

2. ast.literal_eval raises an exception if the input isn't a valid Python datatype, so the code won't be executed if it's not.

Answer 2

ast.literal_eval() expects a string. Convert webhook_data to string with str() and it will work.

webhook_data = {"side": "BUY","key": "8234023409fa3242309sdfasdf903024917325"}

def parse_webhook(webhook_data):

    """
    :param webhook_data: POST data from tradingview, as a string.
    :return: Dictionary version of string.
    """

    data = ast.literal_eval(str(webhook_data)) # this fixes the issue
    return data

---

print(parse_webhook(webhook_data))

Output:

{'side': 'BUY', 'key': '8234023409fa3242309sdfasdf903024917325'}