Simple interactive BASIC interpreter written in Python
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A BASIC Interpreter - Program like it's 1979!


A simple interactive BASIC interpreter written in Python 3. It is based heavily on material in the excellent book Writing Interpreters and Compilers for the Raspberry Pi Using Python by Anthony J. Dos Reis. However, I have had to adapt the Python interpreter presented in the book, both to work with the BASIC programming language and to produce an interactive command line interface. The interpreter therefore adopts the key techniques for interpreter and compiler writing, the use of a lexical analysis stage followed by a recursive descent parser which implements the context free grammar representing the target programming language.

The interpreter is a homage to the home computers of the early 1980s, and when executed, presents an interactive prompt ('>') typical of such a home computer. Commands to run, list, save and load BASIC programs can be entered at the prompt as well as program statements themselves.

The BASIC dialect that has been implemented is slightly simplified, and naturally avoids machine specific instructions, such as those concerned with sound and graphics for example. It allows a limited range of arithmetic expressions composed of multiplication, division, addition and subtraction (including the use of parentheses to change precedence), thus:

> 10 PRINT 2 * 3
> 20 PRINT 20 / 10
> 30 PRINT 10 + 10
> 40 PRINT 10 - 10

Additional numerical operations may be performed using numeric functions (see below).

There is reasonably comprehensive error checking. Syntax errors will be picked up and reported on by the lexical analyser as statements are entered. Runtime errors will highlight the cause and the line number of the offending statement.

The interpreter can be invoked as follows:

$ python


Programs may be listed using the LIST command:

10 LET I = 10

A program is executed using the RUN command:


A program may be saved to disk using the SAVE command. Not that the full path must be specified within double quotes:

> SAVE "C:\path\to\my\file"
Program written to file

Saving is achieved by pickling the Python object that represents the BASIC program, i.e. the saved file is not a textual copy of the program statements.

The program may be re-loaded (i.e. unpickled) from disk using the LOAD command, again specifying the full path using double quotes:

> LOAD "C:\path\to\my\file"
Program read from file

Since loading is performed by unpickling the program object from a file, only BASIC programs previously saved by the interpreter may be loaded.

Individual program statements may be deleted by entering their line number only:

> 10 PRINT "Hello"
> 20 PRINT "Goodbye"
10 PRINT "Hello"
20 PRINT "Goodbye"
> 10
20 PRINT "Goodbye"

The program may be erased entirely from memory using the NEW command:

> 10 LET I = 10
10 LET I = 10

Finally, it is possible to terminate the interpreter by issuing the EXIT command:


On occasion, it might be necessary to force termination of a program and return to the interpreter, for example, because it is caught in an infinite loop. This can be achieved by using Ctrl-C to force the program to stop:

> 10 PRINT "Hello"
> 20 GOTO 10
Program terminated
10 PRINT "Hello"
20 GOTO 10

Programming language constructs

Statement structure

As per usual in old school BASIC, all program statements must be prefixed with a line number which indicates the order in which the statements may be executed. There is no renumber command to allow all line numbers to be modified. A statement may be modified or replaced by re-entering a statement with the same line number:

> 10 LET I = 10
10 LET I = 10
> 10 LET I = 200
10 LET I = 200


Variable types follow the typical BASIC convention. Simple variables may contain either strings or numbers (the latter may be integers or floating point numbers). Likewise array variables may contain arrays of either strings or numbers, but they cannot be mixed in the same array.

Note that all keywords and variable names are case insensitive (and will be converted to upper case internally by the lexical analyser). String literals will retain their case however. There is no inherent limit on the length of variable names or string literals, this will be dictated by the limitations of Python. The range of numeric values is also dependent upon the underlying Python implementation. Note that variable names must only consist of alphabetic characters, not numbers or special characters (e.g. MYVAR5 and MY_VAR are invalid). Alphanumeric variable names that include special characters such as underscores are a possible future enhancement.

Numeric variables have no suffix, whereas string variables are always suffixed by '$'. Note that 'I' and 'I$' are considered to be separate variables. Note that string literals must always be enclosed within double quotes (not single quotes). Using no quotes will result in a syntax error.

Array variables are defined using the DIM statement, which explicitly lists how many dimensions the array has, and the sizes of those dimensions:

> 10 DIM A(3, 3, 3)

Note that the index of each dimension always starts at zero. So in the above example, valid index values for array A will be 0, 1 or 2 for each dimension. Arrays may have a maximum of three dimensions.

As for simple variables, a string array has its name suffixed by a '$' character, while a numeric array does not carry a suffix. An attempt to assign a string value to a numeric array or vice versa will generate an error.

Note that the same variable name cannot be used for both an array and a simple variable. For example, the variables I$ and I$(10) should not be used within the same program, the results may be unpredictable. Also, array variables with the same name but different dimensionality are treated as the same. For example, using a DIM statement to define I(5) and then a second DIM statement to define I(5, 5) will result in the second definition (the two dimensional array) overwriting the first definition (the one dimensional array).

Array values may be used within any expression, such as in a PRINT statement for string values, or in any numerical expression for number values. However, you must be specific about which array element you are referencing, using the correct number of in-range indexes. If that particular array value has not yet been assigned, then an error message will be printed.

> 10 DIM MYARRAY(2, 2, 2)
> 20 LET MYARRAY(0, 1, 0) = 56
> 30 PRINT A(0, 1, 0)
> 30 PRINT A(0, 0, 0)
Empty array value returned in line 30

As in all implementations of BASIC, there is no garbage collection (not surprising since all variables have global scope)!

Program constants

Constants may be defined through the use of the DATA statement. They may consist of numeric or string values and are declared in a comma separated list:

> 10 DATA 56, "Hello", 78

TO DO - Implementation of READ statement.


The REM statement is used to indicate a comment, and occupies an entire statement. It has no effect on execution:


Note that comments will be automatically normalised to upper case.

Stopping a program

The STOP statement may be used to cease program execution:

> 10 PRINT "one"
> 20 STOP
> 30 PRINT "two"

A program will automatically cease execution when it reaches the final statement, so a STOP may not be necessary. However a STOP will be required if subroutines have been defined at the end of the program, otherwise execution will continue through to those subroutines without a corresponding subroutine call. This will cause an error when the RETURN statement is processed and the interpreter attempts to return control back to the caller.


Assignment may be made to numeric simple variables (which can contain either integers or floating point numbers) and string simple variables (string variables are distinguished by their dollar suffix). The interpreter will enforce this division between the two types:

> 10 LET I = 10
> 20 LET I$ = "Hello"

The LET keyword is also optional:

> 10 I = 10

Array variables may also have values assigned to them. The indexes can be derived from numeric expressions:

> 10 DIM NUMS(3, 3)
> 20 DIM STRS$(3, 3)
> 30 LET INDEX = 0
> 50 LET STRS$(INDEX, INDEX) = "hello"

Attempts to assign the wrong type (number or string) to a numeric or string array, attempts to assign a value to an array by specifying the wrong number of dimensions, and attempts to assign to an array using an out of range index, will all result in an error.

Printing to standard output

The PRINT statement is used to print to the screen:

> 10 PRINT 2 * 4
> 10 PRINT "Hello"

Multiple items may be printed by providing a comma separated list. The items in the list will be printed immediately after one another, so spaces must be inserted if these are required:

> 10 PRINT 345, " hello ", 678
345 hello 678

A blank line may be printed by using the PRINT statement without arguments:

> 10 PRINT "Here is a blank line:"
> 20 PRINT
> 30 PRINT "There it was"
Here is a blank line:

There it was

Unconditional branching

Like it or loath it, the GOTO statement is an integral part of BASIC, and is used to transfer control to the statement with the specified line number:

> 10 PRINT "Hello"
> 20 GOTO 10

Subroutine calls

The GOSUB statement is used to generate a subroutine call. Control is passed back to the program at the next statement after the call by a RETURN statement at the end of the subroutine:

> 10 GOSUB 100
> 20 PRINT "This happens after the subroutine"
> 30 STOP
> 110 PRINT "This happens in the subroutine"
> 120 RETURN
This happens in the subroutine
This happens after the subroutine

Note that without use of the STOP statement, execution will run past the last statement of the main program (line 30) and will re-execute the subroutine again (at line 100).

Subroutines may be nested, that is, a subroutine call may be made within another subroutine.

A subroutine may also be called using the ON-GOSUB statement (see Conditional branching below).


Bounded loops are achieved through the use of FOR-NEXT statements. The loop is controlled by a numeric loop variable that is incremented or decremented from a start value to an end value. The loop terminates when the loop variable reaches the end value. The loop variable must also be specified in the NEXT statement at the end of the loop.

> 10 FOR I = 1 TO 3
> 20 PRINT "hello"
> 30 NEXT I

Loops may be nested within one another.

The STEP statement allows the loop variable to be incremented or decremented by a specified amount. For example, to count down from 5 in steps of -1:

> 10 FOR I = 5 TO 1 STEP -1
> 20 PRINT I
> 30 NEXT I

Note that the start value, end value and step value need not be integers, but can be floating point numbers as well. If the loop variable was previously assigned in the program, its value will be replaced by the start value, it will not be evaluated.

Conditional branching

Conditional branches are implemented using the IF-THEN-ELSE statement. The expression is evaluated and the appropriate jump made depending upon the result of the evaluation.

> 20 LET I = 10
> 30 LET J = 20
> 40 IF I > J THEN 50 ELSE 70
> 50 PRINT I
> 60 GOTO 80
> 70 PRINT J

Note that the ELSE clause is optional and may be omitted. In this case, the THEN branch is taken if the expression evaluates to true, otherwise the following statement is executed.

It is also possible to call a subroutine depending upon the result of a relational expression using the ON-GOSUB statement. If the expression evaluates to true, then the subroutine is called, otherwise execution continues to the next statement without making the call:

> 10 LET I = 10
> 20 LET J = 5
> 30 ON I > J GOSUB 100
> 40 STOP
> 110 PRINT "I is greater thn J"
> 120 RETURN
I is greater than J

Allowable relational operators are:

  • '=' (equal, note that in BASIC the same operator is used for assignment)
  • '<' (less than)
  • '>' (greater than)
  • '<=' (less than or equal)
  • '>=' (greater than or equal)
  • '<>' (not equal)

User input

The INPUT statement is used to solicit input from the user:

> 10 INPUT A
> 20 PRINT A
? 22

The default input prompt of '? ' may be changed by inserting a prompt string, which must be terminated by a colon, thus:

> 10 INPUT "Input a number - ": A
> 20 PRINT A
Input a number - 22

Multiple items may be input by supplying a comma separated list. Input variables will be assigned to as many input values as supplied at run time. If there are more input values supplied than input variables, the excess input values will be ignored. Conversely, if not enough input values are supplied, then the excess input variables will not be initialised (and will trigger an error if an attempt is made to evaluate those variables later in the program).

Further, numeric input values must be valid numbers (integers or floating point), and must be unquoted. String input values must be surrounded by double quotes:

> 10 INPUT "Num, Str, Num: ": A, B$, C
> 20 PRINT A, B$, C
Num, Str, Num: 22, " hello ", 33
22 hello 33

A mismatch between the input value and input variable type will trigger an error.

TO DO - User input can only be assigned to simple variables, need to extend to array variables.

Numerical functions

Selected numerical functions are provided, and may be used with any numeric expression. For example, the square root function, SQR, can be applied expressions consisting of both literals and variables:

> 10 LET I = 6
> 20 PRINT SQR(I - 2)

Allowable numeric functions are:

  • ABS(x) - Calculates the absolute value of x

  • ATN(x) - Calculates the arctangent of x

  • COS(x) - Calculates the cosine of x, where x is an angle in radians

  • EXP(x) - Calculates the exponential of x, e^x where e=2.718281828

  • LOG(x) - Calculates the natural logarithm of x

  • POW(x, y) - Calculates x to the power y

  • RND - Generates a pseudo random number N, where 0 <= N < 1. Can be reset using the RANDOMIZE instruction with an optional seed value: e.g.

> 10 RANDOMIZE 100 
  • SIN(x) - Calculates the sine of x, where x is an angle in radians

  • SQR(x) - Calculates the square root of x

  • TAN(x) - Calculates the tangent of x, where x is an angle in radians

Example programs

A number of example BASIC programs have been supplied in the repository:

  • regression.bas - A program to exercise the key programming language constructs in such a way as to allow verification that the interpreter is functioning correctly.

  • factorial.bas - A simple BASIC program to take a number, N, as input from the user and calculate the corresponding factorial N!.

  • rock_scissors_paper.bas - A BASIC implementation of the rock-paper-scissors game.

Note that you cannot simply load these programs from the text files. They must be entered line by line into the interpreter. The program can then be saved and reloaded using the SAVE and LOAD commands as described above. Of course, this is no more inconvenient than saving a program to cassette tape and reloading it, as we all would have done in the 1980s!


ABS(numerical-expression) - Calculates the absolute value of the result of numerical-expression

ATN(numerical-expression) - Calculates the arctangent value of the result of numerical-expression

COS(numerical-expression) - Calculates the cosine value of the result of numerical-expression

DATA(expression-list) - Defines a list of string or numerical values

DIM array-variable(dimensions) - Defines a new array variable

EXIT - Exits the interpreter

EXP(numerical-expression) - Calculates the exponential value of the result of numerical-expression

FOR loop-variable = start-value TO end-value [STEP increment] - Bounded loop

GOSUB line-number - Subroutine call

GOTO line-number - Unconditional branch

IF relational-expression THEN line-number [ELSE line-number] - Conditional branch

INPUT [input-prompt:] variable-list - Processes user input presented as a comma separated list

[LET] variable = numeric-expression | string-expression - Assigns a value to a simple variable or array variable

LIST - Lists the program

LOAD filename - Loads a program from disk

LOG(numerical-expression) - Calculates the natural logarithm value of the result of numerical-expression

NEW - Clears the program from memory

NEXT loop-variable - See FOR statement

ON relational-expression GOSUB line-number - Conditional subroutine call

POW(base, exponent) - Calculates the result of raising the base to the power of the exponent

PRINT print-list - Prints a comma separated list of literals or variables

RANDOMIZE [numeric-expression] - Resets random number generator to an unpredictable sequence. With optional seed (numeric expression), the sequence is predictable.

REM comment - Internal program documentation

RETURN - Return from a subroutine

RND - Generates a pseudo random number N, where 0 <= N < 1

RUN - Runs the program

SAVE filename - Saves a program to disk

SIN(numerical-expression) - Calculates the sine value of the result of numerical-expression

SQR(numerical-expression) - Calculates the square root of the expression

STOP - Terminates a program

TAN(numerical-expression) - Calculates the tangent value of the result of numerical-expression


The interpreter is implemented using the following Python classes:

  • - This implements the tokens that are produced by the lexical analyser. The class mostly defines token categories and provides a simple token pretty printing method.

  • - This class implements the lexical analyser. Lexical analysis is performed on one statement at a time, as each statement is entered into the interpreter.

  • - This class implements a parser for individual BASIC statements. This is somewhat inefficient in that statements, for example those in a loop, must be re-parsed every time they are executed. However, such a model allows us to develop an interactive interpreter where statements can be gradually added to the program between runs. Since the parser is oriented to the processing of individual statements, it uses a signalling mechanism (using FlowSignal objects) to its caller indicate when program level actions are required, such as recording the return address following a subroutine jump. However, the parser does maintain a symbol table (implemented as a dictionary) in order to record the value of variables as they are assigned.

  • - This class implements an actual basic program, which is represented as a dictionary. Dictionary keys are statement line numbers and the corresponding value is the list of tokens that make up the statement with that line number. Statements are executed by calling the parser to parse one statement at a time. This class maintains a program counter, an indication of which line number should be executed next. The program counter is incremented to the next line number in sequence, unless executed a statement has resulted in a branch. The parser indicates this by signalling to the program object that calls it using a FlowSignal object.

  • - This class provides the interface to the user. It allows the user to both input program statements and to execute the resulting program. It also allows the user to run commands, for example to save and load programs, or to list them.

  • - Implements a FlowSignal object that allows the parser to signal a change in control flow. For example, as the result of a jump defined in the statement just parsed (GOTO, conditional branch evaluation), a loop decision, a subroutine call, or program termination. This paradigm of using the parser to simply parse individual statements, the Program object to make control flow decisions and to track execution, and a signalling mechanism to allow the parser to signal control flow changes to the Program object, is used consistently throughout the implementation.

Open issues

  • It is not possible to renumber a program. This would require considerable extra functionality.
  • Negative values are printed with a space (e.g. '- 5') in program listings because of tokenization. This does not affect functionality.
  • Note that variable names must only consist of alphabetic characters, not numbers or special characters (e.g. MYVAR5 and MY_VAR are invalid). Alphanumeric variable names that include special characters such as underscores are a possible future enhancement.
  • Decimal values less than zero must be expressed with a leading zero (i.e. 0.34 rather than .34)
  • User input values cannot be directly assigned to array variables in an INPUT statement