Token metadata and URI in Blockchain / Solidity - Mini Project: Build & Apply

Token metadata and URI

📖 Scenario: You are creating a simple blockchain token system. Each token has metadata like name, description, and an image URI. This metadata helps wallets and apps show useful info about the token.

🎯 Goal: Build a Python dictionary to store token metadata and a URI string. Then create a new dictionary that combines them, and finally print the full token info.

📋 What You'll Learn

Create a dictionary called token_metadata with keys 'name', 'description', and 'image' and exact values.

Create a string variable called token_uri with the exact value.

Create a new dictionary called full_token_info that includes all token_metadata plus the token_uri under key 'uri'.

Print the full_token_info dictionary.

💡 Why This Matters

🌍 Real World

Token metadata and URI are used in blockchain tokens like NFTs to provide details that wallets and marketplaces show to users.

💼 Career

Understanding how to manage token metadata is important for blockchain developers working on smart contracts, NFT platforms, and crypto wallets.

1

Create token metadata dictionary

Create a dictionary called token_metadata with these exact entries: 'name': 'CryptoKitty', 'description': 'A cute blockchain cat', and 'image': 'https://example.com/cat.png'.

Blockchain / Solidity

# Create the token_metadata dictionary with name, description, and image
# Your code here

Hint

Use curly braces {} to create a dictionary. Put keys and values inside with colons.

2

Create token URI string

Create a string variable called token_uri and set it to 'https://example.com/token/123'.

Blockchain / Solidity

token_metadata = {
    'name': 'CryptoKitty',
    'description': 'A cute blockchain cat',
    'image': 'https://example.com/cat.png'
}
# Create the token_uri string
# Your code here

Hint

Use an equals sign = to assign the string to the variable token_uri.

3

Combine metadata and URI into full token info

Create a new dictionary called full_token_info that contains all key-value pairs from token_metadata plus a new key 'uri' with the value from token_uri.

Blockchain / Solidity

token_metadata = {
    'name': 'CryptoKitty',
    'description': 'A cute blockchain cat',
    'image': 'https://example.com/cat.png'
}
token_uri = 'https://example.com/token/123'
# Create full_token_info dictionary combining token_metadata and token_uri
# Your code here

Hint

Use dictionary unpacking {**token_metadata, 'uri': token_uri} to combine dictionaries.

4

Print the full token info

Write a print statement to display the full_token_info dictionary.

Blockchain / Solidity

token_metadata = {
    'name': 'CryptoKitty',
    'description': 'A cute blockchain cat',
    'image': 'https://example.com/cat.png'
}
token_uri = 'https://example.com/token/123'
full_token_info = {**token_metadata, 'uri': token_uri}
# Print the full_token_info dictionary
# Your code here

Hint

Use print(full_token_info) to show the dictionary on screen.

Practice

1. What is the main purpose of token metadata in blockchain tokens?

easy

A. To mine new tokens automatically

B. To describe the token and link to its details

C. To execute smart contract functions

D. To store the token's private key

Solution

1. Step 1: Understand token metadata role

   Token metadata provides descriptive information about the token, such as its name, description, and image link.

2. Step 2: Differentiate from other blockchain functions

   Metadata is not about private keys, mining, or executing contracts but about describing the token for users and apps.

3. Final Answer:

   To describe the token and link to its details -> Option B

4. Quick Check:

   Token metadata = description and details link [OK]

Hint: Metadata describes tokens, not keys or mining [OK]

Common Mistakes:

• Confusing metadata with private keys
• Thinking metadata controls token creation
• Mixing metadata with smart contract code

2. Which of the following is the correct way to specify a token URI in a smart contract?

easy

A. bool tokenURI = true;

B. int tokenURI = 12345;

C. string public tokenURI = "https://example.com/token/1.json";

D. address tokenURI = 0x123;

Solution

1. Step 1: Identify the data type for URI

   A URI is a web address, so it should be stored as a string in the contract.

2. Step 2: Check the options for correct syntax

   string public tokenURI = "https://example.com/token/1.json"; uses a string with a valid URL format, while others use wrong types like int, bool, or address.

3. Final Answer:

   string public tokenURI = "https://example.com/token/1.json"; -> Option C

4. Quick Check:

   URI is a string URL [OK]

Hint: URIs are strings, not numbers or booleans [OK]

Common Mistakes:

• Using int or bool for URI
• Confusing address type with URI
• Missing quotes around the URI string

3. Given the following Solidity snippet, what will tokenURI(1) return if _tokenURIs[1] is set to "https://example.com/metadata/1.json"?

mapping(uint256 => string) private _tokenURIs;

function tokenURI(uint256 tokenId) public view returns (string memory) {
    return _tokenURIs[tokenId];
}

medium

A. An error because tokenURI is private

B. tokenId

C. Empty string

D. "https://example.com/metadata/1.json"

Solution

1. Step 1: Understand the mapping and function

   The mapping stores strings for each tokenId. The function returns the string stored for the given tokenId.

2. Step 2: Check the value for tokenId 1

   Since _tokenURIs[1] is set to "https://example.com/metadata/1.json", calling tokenURI(1) returns that string.

3. Final Answer:

   "https://example.com/metadata/1.json" -> Option D

4. Quick Check:

   Mapping lookup returns stored URI string [OK]

Hint: Mapping returns stored URI for tokenId [OK]

Common Mistakes:

• Thinking tokenURI is private and inaccessible
• Confusing tokenId with URI string
• Assuming function returns tokenId instead of URI

4. Identify the error in this Solidity function that tries to set a token URI:

mapping(uint256 => string) private _tokenURIs;

function setTokenURI(uint256 tokenId, string memory uri) public {
    _tokenURIs[tokenId] = uri;
}

medium

A. The function lacks access control and allows anyone to set URIs

B. The mapping type is incorrect for storing strings

C. The function should return a string

D. The parameter uri should be declared as calldata

Solution

1. Step 1: Review function purpose and security

   The function sets a URI for a tokenId but has no restriction on who can call it.

2. Step 2: Identify missing access control

   Without access control (like onlyOwner), anyone can change token URIs, which is a security risk.

3. Final Answer:

   The function lacks access control and allows anyone to set URIs -> Option A

4. Quick Check:

   Missing access control = security risk [OK]

Hint: Always add access control to setter functions [OK]

Common Mistakes:

• Thinking mapping type is wrong for strings
• Expecting setter functions to return values
• Ignoring security risks of public setters

5. You want to create a function that returns a JSON metadata URI for a token, but if the token has no specific URI set, it should return a default URI. Which Solidity code snippet correctly implements this logic?

mapping(uint256 => string) private _tokenURIs;
string private _defaultURI = "https://example.com/default.json";

function tokenURI(uint256 tokenId) public view returns (string memory) {
    // Fill in the blank
}

hard

A. return bytes(_tokenURIs[tokenId]).length > 0 ? _tokenURIs[tokenId] : _defaultURI;

B. if (_tokenURIs[tokenId] == null) { return _defaultURI; } else { return _tokenURIs[tokenId]; }

C. return _tokenURIs[tokenId] || _defaultURI;

D. return _tokenURIs[tokenId] == "" ? _tokenURIs[tokenId] : _defaultURI;

Solution

1. Step 1: Check how to test if string is empty in Solidity

   Solidity strings can't be compared to null or use || operator; use bytes length to check emptiness.

2. Step 2: Verify correct conditional expression

   return bytes(_tokenURIs[tokenId]).length > 0 ? _tokenURIs[tokenId] : _defaultURI; uses bytes(_tokenURIs[tokenId]).length > 0 to check if URI exists, else returns default URI.

3. Final Answer:

   return bytes(_tokenURIs[tokenId]).length > 0 ? _tokenURIs[tokenId] : _defaultURI; -> Option A

4. Quick Check:

   Check string length with bytes() for empty [OK]

Hint: Use bytes().length to check empty string in Solidity [OK]

Common Mistakes:

• Using null or empty string comparison directly
• Using || operator for strings
• Returning wrong URI when none set