ValueError: malformed node or string in Python [Solved]

# ValueError: malformed node or string in Python [Solved]

The Python "ValueError: malformed node or string" occurs for 3 main reasons:

1. Passing a value that is not a string to the ast.literal_eval() method.
2. Passing a JSON string to the method (use json.loads() instead).
3. Passing a string that is syntactically incorrect to the method.

To solve the error, use the str() class to convert the value to a string or make sure to only call the ast.literal_eval() method with strings.

Here is an example of how the error occurs.

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import ast

a_dict = {
    'first': 'bobby',
    'last': 'hadz',
}

# ⛔️ ValueError: malformed node or string: {'first': 'bobby', 'last': 'hadz'}
print(ast.literal_eval(a_dict))

We called the ast.literal_eval method with a dictionary instead of a string which caused the error.

The method evaluates a string that contains a Python literal.

The string may consist of the following objects:

• strings
• bytes
• numbers
• tuples
• lists
• dictionaries
• sets
• booleans
• None

Note that the method expects to get called with the string representation of the given object, not the object itself (not a dictionary, list or set).

Here is an example of using the ast.literal_eval() method correctly.

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import ast

a_str = '''{
    "first": "bobby",
    "last": "hadz"
}'''

result = ast.literal_eval(a_str)


print(result)  # 👉️ {'first': 'bobby', 'last': 'hadz'}
print(type(result))  # 👉️ <class 'dict'>

Notice that we passed a string to the method and not a dictionary.

The string in the example is a valid dictionary, so it gets parsed to a native Python dict object by the literal_eval() method.

# Converting the object to a string in the call to ast.literal_eval()

One way to solve the error is to use the str() class to convert the object to a string in the call to the ast.literal_eval() method.

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import ast

a_dict = {
    'first': 'bobby',
    'last': 'hadz',
}

result = ast.literal_eval(str(a_dict))

print(result)  # 👉️ {'first': 'bobby', 'last': 'hadz'}
print(type(result))  # 👉️ <class 'dict'>

We used the str() class to convert the dictionary to a string in the call to the method.

No error is raised because the method is called with a string argument.

However, the conversion shouldn't be necessary and could lead to performance issues if the given value is a very large dictionary or list.

# Checking if the supplied value is a string before calling literal_eval()

An alternative approach is to use the isinstance() function to check if the value is a string before calling the ast.literal_eval() method.

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import ast

a_dict = {
    'first': 'bobby',
    'last': 'hadz',
}

if isinstance(a_dict, str):
    result = ast.literal_eval(a_dict)
else:
    result = a_dict


print(result)  # 👉️ {'first': 'bobby', 'last': 'hadz'}
print(type(result))  # 👉️ <class 'dict'>

We only call the ast.literal_eval() method if the value is a string.

Otherwise, we set the result variable to the value.

This helps us avoid the redundant conversion to string.

I've written a detailed article on how to convert the string representation of a list to a list.

The code samples also apply to dictionaries.

Note that calling the ast.literal_eval() method with a string that is not a valid Python object would raise an error.

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import ast

a_str = '''{
    "first": "bobby",
    "last": "hadz",
    "hello" "world"
}'''

# ⛔️ ERROR
result = ast.literal_eval(a_str)

Notice that the last key-value pair in the dictionary is not separated by a colon.

Trying to parse an invalid object causes an error.

# Use the json.loads() method if parsing JSON data

Make sure you aren't using the ast.literal_eval() method to parse a JSON string.

Here is an example of how this causes an error.

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import ast

a_str = '''{
    "first": "bobby",
    "last": "hadz",
    "is_programmer": true,
    "salary": null
}'''

# ⛔️ ValueError: malformed node or string on line 4:
result = ast.literal_eval(a_str)

The string is valid JSON, however, it should not be parsed using ast.literal_eval().

JSON strings contain lowercase boolean values (true and false) and Python booleans have a capital first letter (True and False).

Similarly, we have null in JSON vs None in Python.

Instead, you should use the json.loads() method to convert the JSON string to a native Python object.

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import json

a_str = '''{
    "first": "bobby",
    "last": "hadz",
    "is_programmer": true,
    "salary": null
}'''

result = json.loads(a_str)

# 👇️ {'first': 'bobby', 'last': 'hadz', 'is_programmer': True, 'salary': None}
print(result)

print(type(result)) # 👉️ <class 'dict'>

The json.loads() method parses a JSON string into a native Python object.

I've written a detailed article on how to convert a JSON string to a native Python object.

If you need to convert the string representation of an object to a Python object, check out the following article instead.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Print the binary representation of a Number in Python
• Convert a string to a Class object in Python
• Python: Convert string with comma separator and dot to float