Convert JSON string to dictionary in ROBOT using json.loads() - why are triple quotes needed?

Question

I've read that json.loads() can be used by Robot Framework to convert a JSON string to a dictionary in this post: Json handling in ROBOT

So if you define a dictionary-like string like this:

${json_string}    Set Variable    {"key1": "value1", "key2": "value2", "key3": "value3"}

You can then use the following to convert it to a dictionary:

${dict}    Evaluate    json.loads('''${json_string}''')    json

My question is simple - why are the triple quotes needed here to surround the argument?

If single quotes are used an exception is thrown stating a string must be used:

${dict}    Evaluate    json.loads('${json_string}')    json

(Edit) The above is a bad example, it actually works. If double quotes are used, though, it fails for SyntaxError: invalid syntax.

If no quotes at all are used an error occurs that indicates that the variable is a dictionary - but in Robot it isn't a dictionary (TypeError: the JSON object must be str, bytes or bytearray, not dict):

${dict}    Evaluate    json.loads(${json_string})    json

If Robot's Convert To String is used on the ${json_string} variable and then that new variable is passed to the json.loads() method the same TypeError occurs stating a string must be used, not a dictionary - but it has been converted to a string:

${json_string2}    Convert To String    ${json_string}
${dict}    Evaluate    json.loads(${json_string2})    json

What are the triple quotes accomplishing that are not being accomplished by the other two? This seems to be an aspect of Robot framework...

Answer 1

I'll go ahead and follow up in an answer since it's simpler that way, I think does answer your follow-up question in this comment.

I'm not very familiar with Robot Framework but if I understand correctly all it's doing in .robot files when you use ${variable} substitution is simple string templating: so when you pass a ${variable} into an expression, no matter what type the underlying variable is, it always substitutes its string representation, so the expression you're trying to evaluate is literally:

json.loads({'key': "value1', 'key2': 'value2'})

This is why you need the """ (in principle you could use just ", but """ would be much safer). Incidentally, if you converted the above dict to its standard string representation in Python it would not be valid JSON, because JSON requires double-quotes not single-quotes.

The official documentation on this is a little confusing and seems to contradict itself (I think it was not written by a native English speaker which is OK):

When a variable is used in the expressing using the normal ${variable} syntax, its value is replaces before the expression is evaluated. This means that the value used in the expression will be the string representation of the variable value, not the variable value itself. This is not a problem with numbers and other objects that have a string representation that can be evaluated directly, but with other objects the behavior depends on the string representation.

I say it contradicts itself because first is says "its value [is replaced] before the expression is evaluated". But then it says "the value used in the expression will be the string representation of the variable value" (which is not the same as the value itself). The latter explanation seems to be the correct one though.

However, it seems with Evaluate you can also use the syntax $variable to replace the literal value of the variable instead of its string representation:

Starting from Robot Framework 2.9, variables themselves are automatically available in the evaluation namespace. They can be accessed using special variable syntax without the curly braces like $variable. These variables should never be quoted, and in fact they are not even replaced inside strings.

But in your case you wrote:

${json_string}    Set Variable    {"key1": "value1", "key2": "value2", "key3": "value3"}

As I understand it "Set Variable" just stores the value as a string literal. So indeed it should suffice then to run (as you did):

${dict}    Evaluate    json.loads('''${json_string}''')    json

In your final example "Convert To String" is not doing anything, because as you wrote ${json_string} already replaces as a string. You just have to understand that the result of json.loads(${json_string}) is a Python expression where ${json_string} is just replaced literally with the contents of that template variable--it is not the same as passing a str value to json.loads(). The latter I believe may be achievable as

${dict}    Evaluate    json.loads($json_string)    json

at least, going by the docs. But I have not tested this.