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HelloWorld

The source code is found on GitHub at https://github.com/icon-project/samples
The simplest yet completely legitimate SCORE that returns "Hello" in the response message.
from iconservice import *
​
class HelloWorld(IconScoreBase):
​
    def __init__(self, db: IconScoreDatabase) -> None:
        super().__init__(db)
​
    def on_install(self) -> None:
        super().on_install()
​
    def on_update(self) -> None:
        super().on_update()
​
    @external(readonly=True)
    def hello(self) -> str:
        return "Hello"
We will add a little tweak to our HelloWorld.
Give it a name, "HelloWorld"
Print logs. 
Allow it to accept ICX and other tokens.
from iconservice import *
​
TAG = 'HelloWorld'
​
class HelloWorld(IconScoreBase):
​
    def __init__(self, db: IconScoreDatabase) -> None:
        super().__init__(db)
​
    def on_install(self) -> None:
        super().on_install()
​
    def on_update(self) -> None:
        super().on_update()
​
    @external(readonly=True)
    def name(self) -> str:
        return "HelloWorld"
​
    @external(readonly=True)
    def hello(self) -> str:
        Logger.info('Hello, world!', TAG)
        return "Hello"
​
    @payable
    def fallback(self):
        Logger.info('fallback is called', TAG)
​
    @external
    def tokenFallback(self, _from: Address, _value: int, _data: bytes):
        Logger.info('tokenFallabck is called', TAG)
fallback function is added to accept ICX. fallback function is executed when the contract receives a transaction request without data part. Not having data in the transaction means no method name is specified. In such a case, fallback function is invoked if the function is provided. If fallback function is not given, the transaction will fail.
ICX transfer request message does not have data part. So, the default behavior of any contract is rejecting incoming ICX. This design is to prevent any accidental ICX transfer to a contract.
Only functions with @payable decorator are permitted to receive ICX coins, therefore, our fallback function should be also @payable because we want the contract to receive ICX. You don't need to implement anything in the fallback function to handle the ICX transfer. Having payable fallback function declares that the contract is designed to accept ICX coin transfer.
tokenFallback method is added to accept IRC2 tokens. The IRC2 standard mandates the tokenFallback method in the receiving SCORE. Token contract will call the tokenFallback function whenever it transfers token to a contract address.
GitHub
Please refer to Github for complete source code.
Invoke a method from CLI
Below is the T-Bears CLI command to invoke the SCORE's external read-only method. Before issuing the command, don't forget to change the "to" value in the JSON request with an actual SCORE address.
root@07dfee84208e:/tbears# cat call.json 
{
    "jsonrpc": "2.0",
    "method": "icx_call",
    "id": 1,
    "params": {
        "to": "cx3176b5d6cae66a1abbc3ca9070423a5c708834a9", 
        "dataType": "call",
        "data": {
            "method": "hello"
        }
    }
}
root@07dfee84208e:/tbears# tbears call call.json 
response : {
    "jsonrpc": "2.0",
    "result": "Hello",
    "id": 1
}
Run test
Testcase uses python sdk. You need to install python sdk to run the test.
$ pip3 install iconsdk
Go to the tests folder, open test.py, and change the global variables.
$ tree hello_world
hello_world
├── README.md
├── hello_world
│   ├── __init__.py
│   ├── hello_world.py
│   └── package.json
└── tests
    ├── keystore_test1
    └── test.py
Use the actual SCORE addresses. To run the test, keystore_test1 should have a positive amount of IRC2 tokens. Deploy the IRC2 token contract with -k keystore_test1 option, then all the initial token supply will go to the account. If you test on T-Bears, use the default node_uri. If test on other networks, change the node_uri and network_id accordingly.
node_uri = "http://localhost:9000/api/v3"
network_id = 3
hello_world_address = "cx3176b5d6cae66a1abbc3ca9070423a5c708834a9"
token_address = "cx9a4c4229ab2cbd61a5cc051fbbb6ee7e3e3adfac"
Run the test.
$ python3 test.py
....
----------------------------------------------------------------------
Ran 4 tests in 20.369s
​
OK
If you tailed a log, you would see the logs printing.
$ tail -f tbears.log | grep '\[HelloWorld\]'
[INFO|logger.py:332] 2018-10-07 18:10:48,974 > [HelloWorld] Hello, world!
[INFO|logger.py:332] 2018-10-07 18:10:58,194 > [HelloWorld] fallback is called
[INFO|logger.py:332] 2018-10-07 18:10:58,219 > [HelloWorld] tokenFallabck is called
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