add docs on how to cosume graphql with python

docs/how-tos/python_consume.md

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+# Consume GraphQL API using Python
+
+## Preface 
+
+This guide will explain how fetch data from a GraphQL endpoint and map the response to a 
+Pydantic model. 
+We will cover: 
+
+- Defining a Pydantic model in [Define the Pydantic Model](#define-the-pydantic-model)
+- Fetching the data from GraphQL API in [Fetch Data from GraphQL API](#fetch-data-from-graphql-api)
+- Mapping the response to the Pydantic model in [Map Response Pydantic Model](#map-response-to-pydantic-model)
+
+## Dependencies 
+
+This guide will utilize the following dependencies:
+
+- `requests` to send HTTP request to the GraphQL endpoint
+- `pydantic` for validating and parsing the fetched data
+
+## Define the Pydantic Model
+
+Pydantic is a data validation and parsing library in Python. It provides a efficient way to 
+validate and parse data into Python objects. We can use pydantic models to define the structure and type
+of data we intend to fetch from the enpoint. It automatically validates the input data based on 
+the defined type annotations. If the data does not match the expected types, Pydantic raises errors 
+ensuring the integrity of the data. 
+
+To defind a Pydantic model, we define a class with Pydantic's `BaseModel` class as the base class. 
+We can define a `Person` class with the attributes we intend to fetch from the endpoint. 
+
+!!! example "Define Pydantic Model"
+
+    ```python
+    from pydantic import BaseModel
+
+    class Person(BaseModel):
+        id: int
+        first_name: str
+        last_name: str    
+
+    ```
+!!! info
+
+    Refer to the [How to Guide for Python Service docs](https://diamondlightsource.github.io/graph-federation/how-tos/python_service/) to learn more about the GraphQL API structure
+
+## Fetch Data From GraphQL API
+
+To fetch data from a GraphQL API, we use the `requests` library to send a POST request with a GraphQL query.
+We specify the GraphQL query as a string. 
+
+!!! example "Define Pydantic Model"
+
+    ```python
+    import requests
+
+    url = 'http://127.0.0.1:8000/graphql'
+    graphql_query = """
+        query {
+            person {
+                id,
+                firstName,
+                lastName,
+            }
+        }
+    """
+
+    response = requests.post(url, json={'query': graphql_query})
+
+    ```
+## Map Response to Pydantic Model
+
+We can now map the json response we got from the endpoint to a Pydantic model by mapping the response fields to 
+the Pydantic model fields.
+
+!!! example "Define Pydantic Model"
+
+    ```python
+    response_data = response.json()
+
+    # Extract the person data from the response
+    person_data = response_data['data']['person']
+
+    # Map the GraphQL response fields to the Pydantic model fields
+    person = Person(
+        id=person_data['id'],
+        first_name=person_data['firstName'],
+        last_name=person_data['lastName']
+
+    ```
+
+We can define a function `fetch_person` to fetch and map the `Person` data to the Pydantic model.
+
+!!! example "Fetch and Map Person Data"
+
+    ```python
+    def fetch_person():
+        url = 'http://127.0.0.1:8000/graphql'
+        query = """
+            query {
+                person {
+                    id,
+                    firstName,
+                    lastName,
+                }
+            }
+        """
+
+        response = requests.post(url, json={'query': query})
+        response_data = response.json()
+
+        person_data = response_data['data']['person']
+        
+        person = Person(
+            id=person_data['id'],
+            first_name=person_data['firstName'],
+            last_name=person_data['lastName']
+        )
+        return person
+
+    ```
+!!! tip
+
+    Ensure robustness by handling potential validation errors using `pydantic.ValidationError`
+
+Finally, we can use the function to fetch and print the person data: 
+
+!!! example "Fetch data from GraphQL"
+
+    ```python
+    if __name__ == "__main__":
+        person = fetch_person()
+        if person:
+            print(person)
+    ```
+
+If the [Python graphQL service](https://diamondlightsource.github.io/graph-federation/how-tos/python_service/) is running, executing 
+the script should output: 
+
+`id=1 first_name='foo' last_name='bar'`