Add doctest to erc20 example

docs/examples.rst

@@ -316,45 +316,53 @@ The ERC20 (formally `EIP20 <https://github.com/ethereum/EIPs/blob/master/EIPS/ei
 First, create your Python object representing the ERC20 contract.
 For this example, we'll be inspecting the `Unicorns <https://etherscan.io/token/0x89205a3a3b2a69de6dbf7f01ed13b2108b2c43e7>`_ token contract.
 
-.. code-block:: python
+.. doctest::
 
-    import json
+    >>> import json
+    >>> import os
+
+    # Please play nice and don't use this key for any shenanigans, thanks!
+    >>> os.environ['INFURA_API_KEY'] = "4f1a358967c7474aae6f8f4a7698aefc"
+    >>> from web3.auto.infura import w3
+
 
-    ERC20_ABI = json.loads('[{"constant":true,"inputs":[],"name":"name","outputs":[{"name":"","type":"string"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_spender","type":"address"},{"name":"_value","type":"uint256"}],"name":"approve","outputs":[{"name":"","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"totalSupply","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_from","type":"address"},{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transferFrom","outputs":[{"name":"","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"decimals","outputs":[{"name":"","type":"uint8"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"name":"_owner","type":"address"}],"name":"balanceOf","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"symbol","outputs":[{"name":"","type":"string"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transfer","outputs":[{"name":"","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[{"name":"_owner","type":"address"},{"name":"_spender","type":"address"}],"name":"allowance","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"anonymous":false,"inputs":[{"indexed":true,"name":"_from","type":"address"},{"indexed":true,"name":"_to","type":"address"},{"indexed":false,"name":"_value","type":"uint256"}],"name":"Transfer","type":"event"},{"anonymous":false,"inputs":[{"indexed":true,"name":"_owner","type":"address"},{"indexed":true,"name":"_spender","type":"address"},{"indexed":false,"name":"_value","type":"uint256"}],"name":"Approval","type":"event"}]')  # noqa: 501
+    >>> ERC20_ABI = json.loads('[{"constant":true,"inputs":[],"name":"name","outputs":[{"name":"","type":"string"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_spender","type":"address"},{"name":"_value","type":"uint256"}],"name":"approve","outputs":[{"name":"","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"totalSupply","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_from","type":"address"},{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transferFrom","outputs":[{"name":"","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[],"name":"decimals","outputs":[{"name":"","type":"uint8"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"name":"_owner","type":"address"}],"name":"balanceOf","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"symbol","outputs":[{"name":"","type":"string"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[{"name":"_to","type":"address"},{"name":"_value","type":"uint256"}],"name":"transfer","outputs":[{"name":"","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[{"name":"_owner","type":"address"},{"name":"_spender","type":"address"}],"name":"allowance","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"anonymous":false,"inputs":[{"indexed":true,"name":"_from","type":"address"},{"indexed":true,"name":"_to","type":"address"},{"indexed":false,"name":"_value","type":"uint256"}],"name":"Transfer","type":"event"},{"anonymous":false,"inputs":[{"indexed":true,"name":"_owner","type":"address"},{"indexed":true,"name":"_spender","type":"address"},{"indexed":false,"name":"_value","type":"uint256"}],"name":"Approval","type":"event"}]')  # noqa: 501
 
     # Address field should be the checksum address at which the ERC20 contract was deployed
-    erc20 = w3.eth.contract(address='0x89205A3A3b2A69De6Dbf7f01ED13B2108B2c43e7', abi=ERC20_ABI)
+    >>> erc20 = w3.eth.contract(address='0x89205A3A3b2A69De6Dbf7f01ED13B2108B2c43e7', abi=ERC20_ABI)
 
 Fetch various data about the current state of the ERC20 contract.
 
-.. code-block:: python
+.. doctest::
 
-    name = erc20.functions.name().call()
-    # -> "Unicorns"
-    symbol = erc20.functions.symbol().call()
-    # -> "🦄"
-    decimals = erc20.functions.decimals().call()
-    # -> 1
-    total_supply = erc20.functions.totalSupply().call()
-    # -> 2038 (at time of writing)
+    >>> erc20.functions.name().call()
+    'Unicorns'
+    >>> erc20.functions.symbol().call()
+    '🦄'
+    >>> erc20.functions.decimals().call()
+    0
+    >>> erc20.functions.totalSupply().call()
+    2038
 
 Get the balance of an account address. 
 
-.. code-block:: python
+.. doctest::
 
-    balance = erc20.functions.balanceOf('0xD1220A0cf47c7B9Be7A2E6BA89F429762e7b9aDb').call()
-    # -> 1
+    >>> erc20.functions.balanceOf('0xD1220A0cf47c7B9Be7A2E6BA89F429762e7b9aDb').call()
+    1
 
 Calculate the token count from the token balance of an account address.
 
-.. code-block:: python
+.. doctest::
 
-    from decimal import Decimal
+    >>> from decimal import Decimal
 
     # Most applications expect *exact* results, so using the Decimal library,
     # with default precision of 28, is usually sufficient to avoid any rounding error.
-    count = balance / Decimal(10 ** decimals)
-    # -> 1
+    >>> decimals = erc20.functions.decimals().call()
+    >>> balance = erc20.functions.balanceOf('0xD1220A0cf47c7B9Be7A2E6BA89F429762e7b9aDb').call()
+    >>> balance / Decimal(10 ** decimals)
+    Decimal('1')
 
 Balance is *always* an integer in the currency's smallest natural unit (equivalent to 'wei' for ether). Token balance is typically used only for backend calculations.