Ethereum API Request Problem: Using Variables in Parameters
As an Ethereum developer, you probably know how important it is to use the correct parameter names and data types in your requests. However, a common problem is working with variables that need to be passed as parameters. In this article, we will examine why the {{sellPrice}} variable is rejected by the API and propose a solution.
Error “The signature for this request is invalid”
When making an API request in Postman or another tool, you must specify the parameter name (without quotes) followed by the data type (e.g. “{{sellPrice}}”). The exact syntax may vary depending on the API endpoint and its specific requirements. However, a common problem is using variables without quotes.
{{ }} syntax issue
In JavaScript, literal templates ({...}) and string interpolation (${...}) behave differently with variables. If you use {{ }}, Postman will interpret {} as the start of a variable name, which can cause problems if the variable contains special characters or spaces.
For example, consider a request with a parameter like this:
json
POST /api/price HTTP/1.1
Content type: application/json
{
“price”: “{{ sale price }}”,
“symbol”: “{{ symbol }}”
}
If you use the {{ }} syntax to pass the sellPrice variable, Postman will recognize it as a template literal and ignore it in the request body.
Why {{ }} is not valid
To fix this, you need to escape the ${} characters around the variable with double curly braces ({{{...}}}). This tells Postman that the following parts are placeholder values:
json
POST /api/price HTTP/1.1
Content type: application/json
{
"price": "{{ sellprice }}",
"symbol": "{{ symbol }}"
}
Solution
To use the {{sellPrice}} variable in a request, follow these steps:
- Enclose the
{}around the variable with double curly braces ({{{...}}}).
- Leave the rest of the syntax unchanged.
- In the Postman request body, replace
{{ }}with{{{...}}}>.
Here is an updated example:
json
POST /api/price HTTP/1.1
Content type: application/json
{
“price”: “{{ sale price }}”,
“symbol”: “{{ symbol }}”
}
Tips and Variations
- If you are using a templating engine (such as Handlebars or Mustache), you must use the syntax
{{ variable_name }} instead of {}.
- In some cases, you may need to enclose the variable name in single quotes (
') if it contains special characters. For example:
json
POST /api/price HTTP/1.1
Content-type: application/json
{
"price": "'{{ sellPrice }}'",
"symbol": "'{{ symbol }}'"
}
If you follow these steps, you should be able to successfully use the {{sellPrice}} variable in your API request parameters.
Conclusion
Using variables that contain special characters or spaces can cause errors when sending requests in Postman. By enclosing {} around the variable using double curly braces ({{{...}}}>), you can ensure that Postman correctly interprets your values and passes them as required.
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