Style Guide
Introduction
This guide is intended to provide coding conventions for writing Solidity code. This guide should be thought of as an evolving document that will change over time as useful conventions are found and old conventions are rendered obsolete.
Many projects will implement their own style guides. In the event of conflicts, project specific style guides take precedence.
The structure and many of the recommendations within this style guide were taken from python’s pep8 style guide.
The goal of this guide is not to be the right way or the best way to write Solidity code. The goal of this guide is consistency. A quote from python’s pep8 captures this concept well.
Note
A style guide is about consistency. Consistency with this style guide is important. Consistency within a project is more important. Consistency within one module or function is most important.
But most importantly: know when to be inconsistent – sometimes the style guide just doesn’t apply. When in doubt, use your best judgment. Look at other examples and decide what looks best. And don’t hesitate to ask!
Code Layout
Indentation
Use 4 spaces per indentation level.
Tabs or Spaces
Spaces are the preferred indentation method.
Mixing tabs and spaces should be avoided.
Blank Lines
Surround top level declarations in Solidity source with two blank lines.
Yes:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;
contract A {
// ...
}
contract B {
// ...
}
contract C {
// ...
}
No:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;
contract A {
// ...
}
contract B {
// ...
}
contract C {
// ...
}
Within a contract surround function declarations with a single blank line.
Blank lines may be omitted between groups of related one-liners (such as stub functions for an abstract contract)
Yes:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.6.0 <0.9.0;
abstract contract A {
function spam() public virtual pure;
function ham() public virtual pure;
}
contract B is A {
function spam() public pure override {
// ...
}
function ham() public pure override {
// ...
}
}
No:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.6.0 <0.9.0;
abstract contract A {
function spam() virtual pure public;
function ham() public virtual pure;
}
contract B is A {
function spam() public pure override {
// ...
}
function ham() public pure override {
// ...
}
}
Maximum Line Length
Keeping lines under the PEP 8 recommendation to a maximum of 79 (or 99) characters helps readers easily parse the code.
Wrapped lines should conform to the following guidelines.
The first argument should not be attached to the opening parenthesis.
One, and only one, indent should be used.
Each argument should fall on its own line.
The terminating element,
);, should be placed on the final line by itself.
Function Calls
Yes:
thisFunctionCallIsReallyLong(
longArgument1,
longArgument2,
longArgument3
);
No:
thisFunctionCallIsReallyLong(longArgument1,
longArgument2,
longArgument3
);
thisFunctionCallIsReallyLong(longArgument1,
longArgument2,
longArgument3
);
thisFunctionCallIsReallyLong(
longArgument1, longArgument2,
longArgument3
);
thisFunctionCallIsReallyLong(
longArgument1,
longArgument2,
longArgument3
);
thisFunctionCallIsReallyLong(
longArgument1,
longArgument2,
longArgument3);
Assignment Statements
Yes:
thisIsALongNestedMapping[being][set][to_some_value] = someFunction(
argument1,
argument2,
argument3,
argument4
);
No:
thisIsALongNestedMapping[being][set][to_some_value] = someFunction(argument1,
argument2,
argument3,
argument4);
Event Definitions and Event Emitters
Yes:
event LongAndLotsOfArgs(
address sender,
address recipient,
uint256 publicKey,
uint256 amount,
bytes32[] options
);
LongAndLotsOfArgs(
sender,
recipient,
publicKey,
amount,
options
);
No:
event LongAndLotsOfArgs(address sender,
address recipient,
uint256 publicKey,
uint256 amount,
bytes32[] options);
LongAndLotsOfArgs(sender,
recipient,
publicKey,
amount,
options);
Source File Encoding
UTF-8 or ASCII encoding is preferred.
Imports
Import statements should always be placed at the top of the file.
Yes:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;
import "./Owned.sol";
contract A {
// ...
}
contract B is Owned {
// ...
}
No:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;
contract A {
// ...
}
import "./Owned.sol";
contract B is Owned {
// ...
}
Order of Functions
Ordering helps readers identify which functions they can call and to find the constructor and fallback definitions easier.
Functions should be grouped according to their visibility and ordered:
constructor
receive function (if exists)
fallback function (if exists)
external
public
internal
private
Within a grouping, place the view and pure functions last.
Yes:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.7.0 <0.9.0;
contract A {
constructor() {
// ...
}
receive() external payable {
// ...
}
fallback() external {
// ...
}
// External functions
// ...
// External functions that are view
// ...
// External functions that are pure
// ...
// Public functions
// ...
// Internal functions
// ...
// Private functions
// ...
}
No:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.7.0 <0.9.0;
contract A {
// External functions
// ...
fallback() external {
// ...
}
receive() external payable {
// ...
}
// Private functions
// ...
// Public functions
// ...
constructor() {
// ...
}
// Internal functions
// ...
}
Whitespace in Expressions
Avoid extraneous whitespace in the following situations:
Immediately inside parenthesis, brackets or braces, with the exception of single line function declarations.
Yes:
spam(ham[1], Coin({name: "ham"}));
No:
spam( ham[ 1 ], Coin( { name: "ham" } ) );
Exception:
function singleLine() public { spam(); }
Immediately before a comma, semicolon:
Yes:
function spam(uint i, Coin coin) public;
No:
function spam(uint i , Coin coin) public ;
More than one space around an assignment or other operator to align with another:
Yes:
x = 1;
y = 2;
long_variable = 3;
No:
x = 1;
y = 2;
long_variable = 3;
Don’t include a whitespace in the receive and fallback functions:
Yes:
receive() external payable {
...
}
fallback() external {
...
}
No:
receive () external payable {
...
}
fallback () external {
...
}
Control Structures
The braces denoting the body of a contract, library, functions and structs should:
open on the same line as the declaration
close on their own line at the same indentation level as the beginning of the declaration.
The opening brace should be preceded by a single space.
Yes:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;
contract Coin {
struct Bank {
address owner;
uint balance;
}
}
No:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;
contract Coin
{
struct Bank {
address owner;
uint balance;
}
}
The same recommendations apply to the control structures if, else, while,
and for.
Additionally there should be a single space between the control structures
if, while, and for and the parenthetic block representing the
conditional, as well as a single space between the conditional parenthetic
block and the opening brace.
Yes:
if (...) {
...
}
for (...) {
...
}
No:
if (...)
{
...
}
while(...){
}
for (...) {
...;}
For control structures whose body contains a single statement, omitting the braces is ok if the statement is contained on a single line.
Yes:
if (x < 10)
x += 1;
No:
if (x < 10)
someArray.push(Coin({
name: 'spam',
value: 42
}));
For if blocks which have an else or else if clause, the else should be
placed on the same line as the if’s closing brace. This is an exception compared
to the rules of other block-like structures.
Yes:
if (x < 3) {
x += 1;
} else if (x > 7) {
x -= 1;
} else {
x = 5;
}
if (x < 3)
x += 1;
else
x -= 1;
No:
if (x < 3) {
x += 1;
}
else {
x -= 1;
}
Function Declaration
For short function declarations, it is recommended for the opening brace of the function body to be kept on the same line as the function declaration.
The closing brace should be at the same indentation level as the function declaration.
The opening brace should be preceded by a single space.
Yes:
function increment(uint x) public pure returns (uint) {
return x + 1;
}
function increment(uint x) public pure onlyOwner returns (uint) {
return x + 1;
}
No:
function increment(uint x) public pure returns (uint)
{
return x + 1;
}
function increment(uint x) public pure returns (uint){
return x + 1;
}
function increment(uint x) public pure returns (uint) {
return x + 1;
}
function increment(uint x) public pure returns (uint) {
return x + 1;}
The modifier order for a function should be:
Visibility
Mutability
Virtual
Override
Custom modifiers
Yes:
function balance(uint from) public view override returns (uint) {
return balanceOf[from];
}
function shutdown() public onlyOwner {
selfdestruct(owner);
}
No:
function balance(uint from) public override view returns (uint) {
return balanceOf[from];
}
function shutdown() onlyOwner public {
selfdestruct(owner);
}
For long function declarations, it is recommended to drop each argument onto its own line at the same indentation level as the function body. The closing parenthesis and opening bracket should be placed on their own line as well at the same indentation level as the function declaration.
Yes:
function thisFunctionHasLotsOfArguments(
address a,
address b,
address c,
address d,
address e,
address f
)
public
{
doSomething();
}
No:
function thisFunctionHasLotsOfArguments(address a, address b, address c,
address d, address e, address f) public {
doSomething();
}
function thisFunctionHasLotsOfArguments(address a,
address b,
address c,
address d,
address e,
address f) public {
doSomething();
}
function thisFunctionHasLotsOfArguments(
address a,
address b,
address c,
address d,
address e,
address f) public {
doSomething();
}
If a long function declaration has modifiers, then each modifier should be dropped to its own line.
Yes:
function thisFunctionNameIsReallyLong(address x, address y, address z)
public
onlyOwner
priced
returns (address)
{
doSomething();
}
function thisFunctionNameIsReallyLong(
address x,
address y,
address z
)
public
onlyOwner
priced
returns (address)
{
doSomething();
}
No:
function thisFunctionNameIsReallyLong(address x, address y, address z)
public
onlyOwner
priced
returns (address) {
doSomething();
}
function thisFunctionNameIsReallyLong(address x, address y, address z)
public onlyOwner priced returns (address)
{
doSomething();
}
function thisFunctionNameIsReallyLong(address x, address y, address z)
public
onlyOwner
priced
returns (address) {
doSomething();
}
Multiline output parameters and return statements should follow the same style recommended for wrapping long lines found in the Maximum Line Length section.
Yes:
function thisFunctionNameIsReallyLong(
address a,
address b,
address c
)
public
returns (
address someAddressName,
uint256 LongArgument,
uint256 Argument
)
{
doSomething()
return (
veryLongReturnArg1,
veryLongReturnArg2,
veryLongReturnArg3
);
}
No:
function thisFunctionNameIsReallyLong(
address a,
address b,
address c
)
public
returns (address someAddressName,
uint256 LongArgument,
uint256 Argument)
{
doSomething()
return (veryLongReturnArg1,
veryLongReturnArg1,
veryLongReturnArg1);
}
For constructor functions on inherited contracts whose bases require arguments, it is recommended to drop the base constructors onto new lines in the same manner as modifiers if the function declaration is long or hard to read.
Yes:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.7.0 <0.9.0;
// Base contracts just to make this compile
contract B {
constructor(uint) {
}
}
contract C {
constructor(uint, uint) {
}
}
contract D {
constructor(uint) {
}
}
contract A is B, C, D {
uint x;
constructor(uint param1, uint param2, uint param3, uint param4, uint param5)
B(param1)
C(param2, param3)
D(param4)
{
// do something with param5
x = param5;
}
}
No:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.7.0 <0.9.0;
// Base contracts just to make this compile
contract B {
constructor(uint) {
}
}
contract C {
constructor(uint, uint) {
}
}
contract D {
constructor(uint) {
}
}
contract A is B, C, D {
uint x;
constructor(uint param1, uint param2, uint param3, uint param4, uint param5)
B(param1)
C(param2, param3)
D(param4) {
x = param5;
}
}
contract X is B, C, D {
uint x;
constructor(uint param1, uint param2, uint param3, uint param4, uint param5)
B(param1)
C(param2, param3)
D(param4) {
x = param5;
}
}
When declaring short functions with a single statement, it is permissible to do it on a single line.
Permissible:
function shortFunction() public { doSomething(); }
These guidelines for function declarations are intended to improve readability. Authors should use their best judgment as this guide does not try to cover all possible permutations for function declarations.
Mappings
In variable declarations, do not separate the keyword mapping from its
type by a space. Do not separate any nested mapping keyword from its type by
whitespace.
Yes:
mapping(uint => uint) map;
mapping(address => bool) registeredAddresses;
mapping(uint => mapping(bool => Data[])) public data;
mapping(uint => mapping(uint => s)) data;
No:
mapping (uint => uint) map;
mapping( address => bool ) registeredAddresses;
mapping (uint => mapping (bool => Data[])) public data;
mapping(uint => mapping (uint => s)) data;
Variable Declarations
Declarations of array variables should not have a space between the type and the brackets.
Yes:
uint[] x;
No:
uint [] x;
Other Recommendations
Strings should be quoted with double-quotes instead of single-quotes.
Yes:
str = "foo";
str = "Hamlet says, 'To be or not to be...'";
No:
str = 'bar';
str = '"Be yourself; everyone else is already taken." -Oscar Wilde';
Surround operators with a single space on either side.
Yes:
x = 3;
x = 100 / 10;
x += 3 + 4;
x |= y && z;
No:
x=3;
x = 100/10;
x += 3+4;
x |= y&&z;
Operators with a higher priority than others can exclude surrounding whitespace in order to denote precedence. This is meant to allow for improved readability for complex statements. You should always use the same amount of whitespace on either side of an operator:
Yes:
x = 2**3 + 5;
x = 2*y + 3*z;
x = (a+b) * (a-b);
No:
x = 2** 3 + 5;
x = y+z;
x +=1;
Order of Layout
Layout contract elements in the following order:
Pragma statements
Import statements
Interfaces
Libraries
Contracts
Inside each contract, library or interface, use the following order:
Type declarations
State variables
Events
Modifiers
Functions
Note
It might be clearer to declare types close to their use in events or state variables.
Naming Conventions
Naming conventions are powerful when adopted and used broadly. The use of different conventions can convey significant meta information that would otherwise not be immediately available.
The naming recommendations given here are intended to improve the readability, and thus they are not rules, but rather guidelines to try and help convey the most information through the names of things.
Lastly, consistency within a codebase should always supersede any conventions outlined in this document.
Naming Styles
To avoid confusion, the following names will be used to refer to different naming styles.
b(single lowercase letter)B(single uppercase letter)lowercaselower_case_with_underscoresUPPERCASEUPPER_CASE_WITH_UNDERSCORESCapitalizedWords(or CapWords)mixedCase(differs from CapitalizedWords by initial lowercase character!)Capitalized_Words_With_Underscores
Note
When using initialisms in CapWords, capitalize all the letters of the initialisms. Thus HTTPServerError is better than HttpServerError. When using initialisms in mixedCase, capitalize all the letters of the initialisms, except keep the first one lower case if it is the beginning of the name. Thus xmlHTTPRequest is better than XMLHTTPRequest.
Names to Avoid
l- Lowercase letter elO- Uppercase letter ohI- Uppercase letter eye
Never use any of these for single letter variable names. They are often indistinguishable from the numerals one and zero.
Contract and Library Names
Contracts and libraries should be named using the CapWords style. Examples:
SimpleToken,SmartBank,CertificateHashRepository,Player,Congress,Owned.Contract and library names should also match their filenames.
If a contract file includes multiple contracts and/or libraries, then the filename should match the core contract. This is not recommended however if it can be avoided.
As shown in the example below, if the contract name is Congress and the library name is Owned, then their associated filenames should be Congress.sol and Owned.sol.
Yes:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.7.0 <0.9.0;
// Owned.sol
contract Owned {
address public owner;
constructor() {
owner = msg.sender;
}
modifier onlyOwner {
require(msg.sender == owner);
_;
}
function transferOwnership(address newOwner) public onlyOwner {
owner = newOwner;
}
}
and in Congress.sol:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.0 <0.9.0;
import "./Owned.sol";
contract Congress is Owned, TokenRecipient {
//...
}
No:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.7.0 <0.9.0;
// owned.sol
contract owned {
address public owner;
constructor() {
owner = msg.sender;
}
modifier onlyOwner {
require(msg.sender == owner);
_;
}
function transferOwnership(address newOwner) public onlyOwner {
owner = newOwner;
}
}
and in Congress.sol:
// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.7.0;
import "./owned.sol";
contract Congress is owned, tokenRecipient {
//...
}
Struct Names
Structs should be named using the CapWords style. Examples: MyCoin, Position, PositionXY.
Event Names
Events should be named using the CapWords style. Examples: Deposit, Transfer, Approval, BeforeTransfer, AfterTransfer.
Function Names
Functions should use mixedCase. Examples: getBalance, transfer, verifyOwner, addMember, changeOwner.
Function Argument Names
Function arguments should use mixedCase. Examples: initialSupply, account, recipientAddress, senderAddress, newOwner.
When writing library functions that operate on a custom struct, the struct
should be the first argument and should always be named self.
Local and State Variable Names
Use mixedCase. Examples: totalSupply, remainingSupply, balancesOf, creatorAddress, isPreSale, tokenExchangeRate.
Constants
Constants should be named with all capital letters with underscores separating
words. Examples: MAX_BLOCKS, TOKEN_NAME, TOKEN_TICKER, CONTRACT_VERSION.
Modifier Names
Use mixedCase. Examples: onlyBy, onlyAfter, onlyDuringThePreSale.
Enums
Enums, in the style of simple type declarations, should be named using the CapWords style. Examples: TokenGroup, Frame, HashStyle, CharacterLocation.
Avoiding Naming Collisions
single_trailing_underscore_
This convention is suggested when the desired name collides with that of a built-in or otherwise reserved name.
NatSpec
Solidity contracts can also contain NatSpec comments. They are written with a
triple slash (///) or a double asterisk block (/** ... */) and
they should be used directly above function declarations or statements.
For example, the contract from a simple smart contract with the comments added looks like the one below:
// SPDX-License-Identifier: GPL-3.0
pragma solidity >=0.4.16 <0.9.0;
/// @author The Solidity Team
/// @title A simple storage example
contract SimpleStorage {
uint storedData;
/// Store `x`.
/// @param x the new value to store
/// @dev stores the number in the state variable `storedData`
function set(uint x) public {
storedData = x;
}
/// Return the stored value.
/// @dev retrieves the value of the state variable `storedData`
/// @return the stored value
function get() public view returns (uint) {
return storedData;
}
}
It is recommended that Solidity contracts are fully annotated using NatSpec for all public interfaces (everything in the ABI).
Please see the section about NatSpec for a detailed explanation.