.. DO NOT EDIT.
.. THIS FILE WAS AUTOMATICALLY GENERATED BY SPHINX-GALLERY.
.. TO MAKE CHANGES, EDIT THE SOURCE PYTHON FILE:
.. "api/gallery/contracts/tokens/plot_01_erc20_approve_race.py"
.. LINE NUMBERS ARE GIVEN BELOW.

.. only:: html

    .. note::
        :class: sphx-glr-download-link-note

        :ref:`Go to the end <sphx_glr_download_api_gallery_contracts_tokens_plot_01_erc20_approve_race.py>`
        to download the full example code or to run this example in your browser via JupyterLite.

.. rst-class:: sphx-glr-example-title

.. _sphx_glr_api_gallery_contracts_tokens_plot_01_erc20_approve_race.py:


ERC-20 tokens and the approve/transferFrom race (2015)
======================================================

Fabian Vogelsteller and Vitalik Buterin's ERC-20 gave fungible tokens one
interface: ``transfer``, ``balanceOf``, and a two-step delegation, where
``approve(spender, n)`` lets a spender move up to ``n`` tokens with
``transferFrom``. Any wallet or exchange can then handle any token.

``approve`` *overwrites* the allowance. Suppose Alice allowed Bob 100 and
now wants to lower it to 50. Bob sees her transaction waiting in the
mempool and gets a ``transferFrom`` of 100 mined first; her ``approve``
then grants a fresh 50, which he also spends. With the order chosen by the
spender, Alice loses

.. math::

   \text{old} + \text{new} \quad\text{instead of at most}\quad \max(\text{old}, \text{new}).

``decrease_allowance`` changes the allowance relative to what is left, so
if Bob has already spent it, Alice's reduction reverts instead of granting
more.

.. GENERATED FROM PYTHON SOURCE LINES 26-30

.. code-block:: Python

    import matplotlib.pyplot as plt

    import blockchainkit as bk








.. GENERATED FROM PYTHON SOURCE LINES 31-33

Alice lowers Bob's allowance from 100 to 50
-------------------------------------------

.. GENERATED FROM PYTHON SOURCE LINES 33-79

.. code-block:: Python



    def spent_by_bob(fix, bob_first):
        world = bk.contracts.World()
        token = world.deploy("alice", bk.contracts.ERC20, 1_000)
        world.transact("alice", token, "approve", "bob", 100)
        alice_tx = ("decrease_allowance", "bob", 50) if fix else ("approve", "bob", 50)
        if bob_first:
            world.transact("bob", token, "transfer_from", "alice", "bob", 100)
        world.transact("alice", token, *alice_tx)
        # Bob then spends whatever allowance is left.
        remaining = world.view(token, "allowance", "alice", "bob")
        if remaining:
            world.transact("bob", token, "transfer_from", "alice", "bob", remaining)
        return world.view(token, "balance_of", "bob")


    outcomes = {
        (fix, bob_first): spent_by_bob(fix, bob_first)
        for fix in (False, True)
        for bob_first in (False, True)
    }
    print(outcomes)
    assert outcomes[(False, False)] == 50 and outcomes[(False, True)] == 150
    assert outcomes[(True, False)] == 50 and outcomes[(True, True)] == 100

    fig, ax = plt.subplots(figsize=(7, 4))
    labels = ["approve(50)", "decrease_allowance(50)"]
    width = 0.35
    for offset, bob_first, color in ((-width / 2, False, "#16a34a"), (width / 2, True, "#dc2626")):
        heights = [outcomes[(fix, bob_first)] for fix in (False, True)]
        ax.bar(
            [i + offset for i in range(2)],
            heights,
            width,
            color=color,
            label="Bob front-runs" if bob_first else "in Alice's order",
        )
    ax.axhline(100, color="#64748b", linestyle=":", label="what Alice ever allowed at once")
    ax.set_xticks(range(2), labels)
    ax.set(ylabel="tokens Bob ends with", title="Overwriting an allowance invites a race")
    ax.legend()
    fig.tight_layout()

    plt.show()




.. image-sg:: /api/gallery/contracts/tokens/images/sphx_glr_plot_01_erc20_approve_race_001.png
   :alt: Overwriting an allowance invites a race
   :srcset: /api/gallery/contracts/tokens/images/sphx_glr_plot_01_erc20_approve_race_001.png
   :class: sphx-glr-single-img


.. rst-class:: sphx-glr-script-out

 .. code-block:: none

    {(False, False): 50, (False, True): 150, (True, False): 50, (True, True): 100}




.. GENERATED FROM PYTHON SOURCE LINES 80-86

Exercise
--------
Another mitigation is to require the allowance to be zero before it is
set to a new nonzero value: Alice approves 0, then 50. Can Bob still take
150? Write the sequence of transactions and check it with ``World``. A
worked solution is in :doc:`/exercises/contracts`.


.. rst-class:: sphx-glr-timing

   **Total running time of the script:** (0 minutes 0.038 seconds)


.. _sphx_glr_download_api_gallery_contracts_tokens_plot_01_erc20_approve_race.py:

.. only:: html

  .. container:: sphx-glr-footer sphx-glr-footer-example

    .. container:: lite-badge

      .. image:: images/jupyterlite_badge_logo.svg
        :target: ../../../../lite/lab/index.html?path=api/gallery/contracts/tokens/plot_01_erc20_approve_race.ipynb
        :alt: Launch JupyterLite
        :width: 150 px

    .. container:: sphx-glr-download sphx-glr-download-jupyter

      :download:`Download Jupyter notebook: plot_01_erc20_approve_race.ipynb <plot_01_erc20_approve_race.ipynb>`

    .. container:: sphx-glr-download sphx-glr-download-python

      :download:`Download Python source code: plot_01_erc20_approve_race.py <plot_01_erc20_approve_race.py>`

    .. container:: sphx-glr-download sphx-glr-download-zip

      :download:`Download zipped: plot_01_erc20_approve_race.zip <plot_01_erc20_approve_race.zip>`


.. only:: html

 .. rst-class:: sphx-glr-signature

    `Gallery generated by Sphinx-Gallery <https://sphinx-gallery.github.io>`_