HomeCoinsAlgorand (ALGO)PyTeal — Writing Algorand Good Contracts in Python

PyTeal — Writing Algorand Good Contracts in Python

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Shumo Chu

PyTeal is a python language binding for Algorand Good Contracts (ASC) that abstracts away the complexities in writing good contracts. PyTeal is offered as an open-source software for the Algorand group. We invite you to attempt, use, and contribute to PyTeal if you’re concerned with growing and deploying ASC’s in Python.

Within the 2.Zero launch, we added ASC because the excessive efficiency layer-1 good contract primitive to Algorand.

ASC’s are written in a byte-code based mostly stack language referred to as Transaction Execution Approval Language (TEAL). TEAL can analyze and approve transactions however it can not create or change transactions, and because of this returns solely true or false.

TEAL was designed to be considerably restricted in relation to what most the world thinks of when the time period “good contact” is used. The language is a non “turing-complete” language, that doesn’t assist looping however does assist ahead branches, it consists of 30 fundamental directions, it executes as a straight-line program solely utilizing TEAL operations and each ASC is at most 1KB-long.

Nevertheless, there’s important overhead in writing logic utilizing a stack language. There may be additionally the added overhead in needing to study yet one more blockchain language.

PyTeal permits Algorand builders to precise their good contract logic in Python. The PyTeal library then compiles the good contract logic to TEAL supply code for the builders. In a nutshell, PyTeal abstracts away a variety of heavy lifting that’s required when writing TEAL.

Let’s check out a periodic fee PyTeal program.

The core of this instance is discovered between traces 14 and 30 (In traces 6–12, we’re merely defining constants).

Let’s check out periodic_pay_core :

The very first thing this program does is test the kind of transaction. On this case, it’s checking for a pay transaction.

Txn.type_enum() == Int(1)

Beneath is the listing of TypeEnums:


The subsequent factor this system does is test the payment to make it possible for it’s lower than some cheap quantity. This could at all times be performed as a safety and sanity test as a result of in any other case, somebody may ship a transaction that the contract would possibly approve the place all of cash within the contract is wasted on the transaction payment.

Txn.payment() <= tmpl_fee,

So this says, be certain that the transaction payment is lower than or equal to 1000.

Equally,periodic_pay_transfer checks that the transaction set CloseRemainderTo to world zero handle (which suggests the transaction is not going to be closing out the steadiness of the handle), set the receiver of the transaction to the meant receiver, and set the quantity of the transaction to the meant quantity.

As well as, it permits the switch to occur each tmpl_period rounds for tmpl_dur rounds by checking that the Txn.first_valid() is divisable bytmpl_period and Txn.first_valid() and Txn.last_valid() aretmpl_dur away.

Final, We test that the lease is about to restrict replay fee. Which implies that solely one in all these transactions can maintain this lease between the primary and final legitimate rounds. This parameter is a 32-byte string and, mixed with the sender subject, makes the transaction distinctive. If anybody resends the transaction with the identical lease parameter, and sender, throughout the similar window of the primary and final legitimate rounds, the transaction will probably be rejected.

The ultimate primary part of PyTeal code specifies that shut case. When the periodic fee’s time (Txn.first_valid() > tmpl_timeout), then all of the remaining steadiness is closed to tmpl_rcv .

Now that each one the circumstances are declared, we are able to piece collectively all of the circumstances (periodic_pay_core, periodic_pay_transfer , and periodic_pay_close ) in periodic_pay_escrow :

periodic_pay_escrow = And(periodic_pay_core,           

In totality, periodic_pay_escrow is saying, listed below are the circumstances for the core of this fee transaction :

periodic_pay_core :

  • it must be a fee transaction, and it must be lower than or equal than tmpl_fee

AND one of many following:

periodic_pay_transfer :

  • be certain that the the CloseRemainderTo is about to 0, in order that there isn’t an unintentional exit for the funds which can be within the account, test the receiver, test the quantity, test that this transaction occurs each tmpl_periodrounds for tmpl_dur rounds, test that lease is about correctly


periodic_pay_close :

  • test that the CloseRemainderTo subject is about to the receiver, be certain that the receiver subject is now 0, the FirstValid of the transaction has handed the timeout spherical ( tmpl_timeout ) spherical and that the quantity can be 0.

Operating this python code will produce the next TEAL supply:

Now that we’ve seen PyTeal and perceive the way it compiles to TEAL supply, let’s run the instance.

We will probably be utilizing objective to compile TEAL and Algorand Python SDK to assemble and submit the transaction.

Earlier than working this PyTeal code, just remember to generate an arbitrary 32 byte string for the lease parameter and cross in that very same string into the Python SDK code you’re utilizing to signal and ship the logic. You should utilize the one within the PyTeal code above if you’re simply doing a observe run of this code.

  1. Go forward and import the PyTeal code from the gist on the prime of the put up into your editor of selection.
  2. pip3 set up pyteal
  3. python3 periodic_payment.py

Now, assume you could have put in Algorand Python SDK and Algorand command line software objective. We are able to run the next Python code (assuming the PyTeal code is in periodic_payment.py below similar folder):

The code generates output like:

Woohoo! You deployed a periodic fee good contract!

Subsequent, we present some extra fascinating options offered by PyTeal:

PyTeal overloads Python’s arithmetic and comparability operators (+, -, *, /, >, <, >=, >=, ==) in order that PyTeal customers can categorical good contract logic in Python extra naturally. For instance:

Txn.payment() < Int(4000)

checks that the transaction payment of present transaction is decrease than 4000 microAlgos.

Gtxn.quantity(0) == Gtxn.quantity(1) * Int(2)

checks that the quantity of Algos transferred within the first transaction within the group switch is twice the algos transferred within the second transaction.

In TEAL, there are two knowledge varieties: uint64, an unsigned 64-bit integer sort, and bytes, a uncooked bytes sort. Every TEAL operator’s enter have to be correctly typed. In any other case, there will probably be a runtime error if that a part of TEAL code is executed. In PyTeal, we implement a kind checking sub-system that can reject wrongly typed PyTeal applications on the time of developing PyTeal program. For instance,

“”” sort error“””cond = Txn.payment() < Txn.receiver()

Operating this PyTeal program in Python yields an error message:

TealTypeError: Kind error: TealType.bytes whereas anticipated TealType.uint64

It is because Txn.receiver() is typed bytes whereas the overloaded < count on uint64 typed operands on either side.

PyTeal additionally supplies “sugared” larger stage abstractions that captures use patterns to make growing in PyTeal much more “candy”. For instance, we noticed {that a} widespread use sample of TEAL is to change on totally different circumstances when the transaction must be accredited (just like the obtain case and escape case in hash time locked contract). We create a Cond operator in PyTeal to make expressing such sorts of circumstances a lot simpler. For instance:

dutch = Cond([Global.group_size() == Int(5), bid],
[Global.group_size() == Int(4), redeem],
[Global.group_size() == Int(1), wrapup])

Cond takes a sequence of [condition, action] pairs, and evaluates to the primary motion whose related situation evaluates to true. Within the instance above, if the group measurement of the present Algorand atomic switch is 5, bid is evaluated; if the group measurement is 4, redeem is evaluated; if the group measurement is 1, wrapup will probably be evaluated. If the group measurement is none of those above, the generated TEAL code will return an error.

Thanks for giving this a learn! Please think about using and contributing to PyTeal if you’re concerned with growing and deploying Algorand good contracts in Python.

PyTeal GitHub:

PyTeal Doc:

Algorand Consumer Discussion board:

Victor Luchangco, Sam Abbassi, Jason Weathersby, and Derek Leung additionally contributed to this text. The periodic fee contract is from the unique TEAL periodic fee contract by Derek Leung and Max Justicz.

Learn the unique article right here

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