cannot assign a tuple to a variable pine script

Mac Grediser

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27-11-2024

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The "Cannot Assign a Tuple to a Variable" Error in Pine Script

Pine Script, TradingView's proprietary scripting language, is powerful for creating custom indicators and strategies. However, one common error encountered by new and even experienced users is the "cannot assign a tuple to a variable" message. This error typically arises from a misunderstanding of how Pine Script handles data structures and variable assignments.

This article will dissect the error, explain its root cause, and provide clear solutions to resolve it.

Understanding the Problem:

Pine Script doesn't directly support tuples in the same way languages like Python do. The error "cannot assign a tuple to a variable" usually occurs when you try to assign a series of values enclosed in parentheses () to a single variable. Pine Script interprets these parentheses as representing a single value, which might be a complex expression evaluating to a single result, but not as a collection of independent values.

Common Scenarios Leading to the Error:

1. Incorrect Assignment of Multiple Values:

   Let's say you intend to store multiple values, perhaps representing the high, low, open, and close prices of a bar:

   high_low_open_close = (high, low, open, close) //INCORRECT
   

   This code will throw the error. Pine Script doesn't allow assigning multiple values directly to a single variable like this.

2. Confusion with Function Return Values:

   Functions in Pine Script can return multiple values, but they need to be handled correctly. You cannot simply assign the function's return to a single variable if it's designed to return multiple values. Instead, you must assign each returned value to its own variable.

   For example, if a function myFunction() returns two values:

   [value1, value2] = myFunction() //CORRECT
   

   This is the correct way to handle multiple return values from a function. Note the use of square brackets [] to deconstruct the array returned by the function.

3. Misunderstanding of Array/Series Handling:

   Pine Script primarily uses arrays and series. If you want to store multiple values, you must utilize an array or a series.

   high_low_open_close = array.from(high, low, open, close) //CORRECT using an array
   
   //Or using a series, if the values are time-series data:
   high_low_open_close = series(high, low, open, close) //Technically incorrect, needs proper series definition.  Better to use separate series variables.
   

Solutions and Best Practices:

• Use Arrays or Series: Always use arrays or series to store multiple values. Arrays are suitable for collections of data, while series are specifically designed for time-series data in Pine Script.

• Proper Function Handling: If a function returns multiple values, deconstruct the returned array using square brackets [] to assign each value to a separate variable.

• Break Down Complex Assignments: Instead of attempting a single complex assignment, break it down into multiple simpler assignments.

• Check Your Data Types: Ensure you're working with compatible data types. Type mismatches can also lead to errors.

• Consult the Pine Script Documentation: The official Pine Script documentation provides detailed information on data structures and variable assignments.

Example of Correct Usage:

//@version=5
indicator("Multiple Value Handling", overlay=true)

//Correctly using an array
ohlc_array = array.new_float(4)
array.push(ohlc_array, open)
array.push(ohlc_array, high)
array.push(ohlc_array, low)
array.push(ohlc_array, close)

plot(array.get(ohlc_array, 0), color=color.blue) //Plots the open price

//Correctly using separate variables for multiple returns from a custom function (example)
[ma, stddev] = myMovingAverage(close, 20)

plot(ma, color=color.red)
plot(ma + stddev, color=color.green)
plot(ma - stddev, color=color.green)


//Simple function returning multiple values
//@function
myMovingAverage(source, length) =>
    [ta.sma(source, length), ta.stdev(source, length)]

By understanding these concepts and following the best practices outlined above, you can effectively avoid the "cannot assign a tuple to a variable" error in your Pine Script code and write cleaner, more robust trading strategies and indicators. Remember to always consult the official Pine Script documentation for the most accurate and up-to-date information.