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I’m new to Python 3 and have been experimenting with Dictionaries but I’m having problems testing for a specific key and retrieving its related values from nested dictionaries.

I want a user defined input to be checked against nested dictionaries and if the user input is found I then want to collect the details for that item and add it to another dictionary.

E.g.

basketDict = 
shopDict = "Fruit": "Apple": "2", "Banana": "3",
 "Vegetables": "Lettuce": "5", "Potato": "7"
userQuery = input("What food do you want to check for? ")

userQuery = "Apple"

Desired result:

basketDict = "Fruit": "Apple": "2"

I've attempted to use dictionary comprehension to form a new dictionary of only food items (Apple, Banana, Lettuce etc.) but keep running into issues when trying to collect the related category ("Fruit"/"Vegetable" and quantity information from the nested dictionaries.

Here's my (broken) code:

basketDict = 
shopDict = "Fruit": "Apple": "2", "Banana": "3",
 "Vegetables": "Lettuce": "5", "Potato": "7"
shopCheck = 

userQuery = input("What food do you want to check for? ")
for category, food in shopDict.items():
 for each in food:
 shopCheck.append(each)

 if userQuery not in shopCheck:
 print("That's not available.")
 else:
 print(userQuery + " added to basket. ")
 basketDict[category] = [food]

print(basketDict)

This code is enough for your requirements:

for category, food in shopDict.items():
 if userQuery in food:
 basketDict[category] = userQuery: food[userQuery]

But if you want to add new items to the basket, you should update it properly:

for category, food in shopDict.items():
 if userQuery in food:
 basketDict.update(
 category: **basketDict.get(category, ),
 **userQuery: food[userQuery]
 )

You want to merge the actual dictionary of the category (the basket's actual content) with the selected item's key-value.
Generally in python3 if you have 2 dictionaries this is the way to merge them into one:

d1 = "one": 1, "two": 2
d2 = "three": 3
merged = **d1, **d2 # 'one': 1, 'two': 2, 'three': 3

Because ** is used to unpack the dictionary.

Good way to understand it is if you have a function definition with some arguments, then you can call the function with a dictionary that has the argument names as keys. But you have to unpack it, otherwise you would pass the dictionary as the parameter, and not its values.

d = "one": 1, "two": 2
def example(one, two):
 pass

example(d) would throw a TypeError, because it's missing the second positional argument, but example(**d) would work fine

Probably there's a simpler way to do this, but I just fixed your code.
When I run your code, everything inside shopDict was going to basketDict. This is because you wasn't cleaning your shopCheck variable at each for iteration.
And there was a second mistake when adding the elements to basketDict. You need to add only the food element that match the userQuery. Here's the full fixed code:

basketDict = 
shopDict = "Fruit": "Apple": "2", "Banana": "3",
 "Vegetables": "Lettuce": "5", "Potato": "7"

userQuery = input("What food do you want to check for? ")
for category, food in shopDict.items():
 shopCheck = 
 for each in food:
 shopCheck.append(each)

 if userQuery not in shopCheck:
 print("That's not available.")
 else:
 print(userQuery + " added to basket. ")
 basketDict[category] = userQuery:food[userQuery]

print(basketDict)

I'd start by creating a dictionary containing all the items you can buy, whether they're fruit or vegetables. Then I'd simply check if the required item is on that dictionary:

basketDict = 
shopDict = "Fruit": "Apple": "2", "Banana": "3",
 "Vegetables": "Lettuce": "5", "Potato": "7"
allVals = 

for k, subDict in shopDict.items():
 for k1, val in subDict.items():
 allVals[k1] = val


userQuery = input("What food do you want to check for? ")
if userQuery not in allVals.keys():
 print("That's not available.")
else:
 print(userQuery + " added to basket. ")
 basketDict[userQuery] = allVals[userQuery]

print(basketDict)