Simple interactive BASIC interpreter written in Python


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.

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

Although this started of as a personal project, it has been enhanced considerably by some other Github users. You can see them in the list of contributors! It’s very much a group endeavour now.


A limited range of arithmetic expressions are provided. Addition and subtraction have the lowest precedence,
but this can be changed with parentheses.

  • + - Addition
  • - - Subtraction
  • * - Multiplication
  • / - Division
  • MOD (or %) - Modulo
> 10 PRINT 2 * 3
> 20 PRINT 20 / 10
> 30 PRINT 10 + 10
> 40 PRINT 10 - 10
> 50 PRINT 15 MOD 10

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

Not also that ‘+’ does extra duty as a string concatenation operator, while ‘*’ can be used to repeat strings.


Programs may be listed using the LIST command:

10 LET I = 10

The list command can take arguments to refine the line selection listed

LIST 50 Lists only line 50.

LIST 50-100 Lists lines 50 through 100 inclusive.

LIST 50 100 Also Lists lines 50 through 100 inclusive, almost any delimiter
works here.

LIST -100 Lists from the start of the program through line 100 inclusive.

LIST 50- Lists from line 50 to the end of the program.

A program is executed using the RUN command:


A program may be saved to disk using the SAVE command. Note that the full path must be specified within double quotes:

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

The program may be re-loaded from disk using the LOAD command, again specifying the full path using double quotes:

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

When loading or saving, the .bas extension is assumed if not provided. If you are loading a simple name (alpha/numbers only) and in the working dir, quotes can be omitted:

> LOAD regression

will load regression.bas from the current working directory.

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

Multiple statements may appear on one line separated by a colon:

> 10 LET X = 10: PRINT X

NOTE: Currently inline loops are NOT supported

10 FOR I = 1 to 10: PRINT I: NEXT

will need to be decomposed to individual lines.


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 may only consist of alphanumeric characters and underscores. However, they
must all begin with an alphabetic character. For example:

  • MY_VAR
  • MY_VAR6$
  • VAR77(0, 0)

are all valid variable names, whereas:

  • 5_VAR
  • _VAR$
  • 66$

are all invalid.

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, but for
compatibility with some basic dialects the bounds of each dimension will be
expanded by one to enable element access including the len. So in the above example,
valid index values for array A will be 0, 1, 2 or 3
for each dimension. Arrays may have a maximum of three dimensions. Numeric arrays will
be initialised with each element set to zero, while string arrays will be initialised
with each element set to the empty string “”.

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.

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 MYARRAY(0, 1, 0)
> 30 PRINT MYARRAY(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

These values can then later be assigned to variables using the READ statement. Note that the type of the value
(string or numeric) must match the type of the variable, otherwise an error message will be triggered. Therefore,
attention should be paid to the relative ordering of constants and variables. Once the constants on a DATA
statement are used by a READ statement, the next READ
statement will move to the DATA statement with the next higher line number, if there are no more DATA
statements before the end of the program an error will be displayed. This is to ensure that the program is not left
in a state where a variable has not been assigned a value, but nevertheless an attempt to use that variable is
made later on in the program.

Normally each DATA statement is consumed sequently by READ statements however, the RESTORE statement can
be used to override this order and set the line number of the DATA statement that will be used by the next
READ statement. If the line-number used in a RESTORE statement does not refer to a DATA statement an
error will be displayed.

The constants defined in the DATA statement may be consumed using several READ statements or several DATA
statements may be consumed by a single READ statement.:

> 10 DATA 56, "Hello", 78
> 30 PRINT FIRSTNUM, " ", S$
> 60 DATA "Another "
> 70 DATA "Line "
> 80 DATA "of "
> 90 DATA "Data"
> 100 RESTORE 10
> 120 PRINT S$," ",A$,B$,C$,D$
56 Hello
Hello Another Line of Data

It is a limitation of this BASIC dialect that it is not possible to assign constants directly to array variables
within a READ statement, only simple variables.


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


Stopping a program

The STOP statement may be used to cease program execution. The command END has the same effect.

> 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

> 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 (or to a file, see File I/O below):

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

Multiple items may be printed by providing a semicolon 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

A print statement terminated by a semicolon will not include a CR/LF.

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


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.

After the completion of the loop, the loop variable value will be the end value + step value (unless
the loop is exited using a GOTO statement).


Conditionals are implemented using the IF-THEN-ELSE statement. The expression is evaluated and the appropriate
statements executed depending upon the result of the evaluation. If a positive integer is supplied as
the THEN or the ELSE statement, a branch will be performed to the indicated line number.

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 next statement is executed.

Conditional branching example:

> 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

The following code segment is equivalent to the segment above:

> 20 LET I = 10
> 30 LET J = 20

A THEN or ELSE can be supplied multiple statements if they are separated by a colon “:”.

> 20 LET I = 10
> 30 LET J = 20
> 50 PRINT L

Note that should an IF-THEN-ELSE stmt be used in a THEN code block or multiple IF-THEN-ELSE statements
are used in either a single THEN or ELSE code block, the block grouping is ambiguous and logical processing
may not function as expected. There is no ambiguity when single IF-THEN-ELSE statements are placed within ELSE


> 100 IF I > J THEN IF J >= 100 THEN PRINT "I > 100" else PRINT "Not clear which **IF** this belongs to"

Not Ambiguous:

> 100 IF I < J THEN PRINT "I is less than J" ELSE IF J > 100 THEN PRINT "I > 100" ELSE PRINT "J <= 100"

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)

The logical operators AND and OR are also provided to allow you to join two or more expressions. The NOT operator can also be given before an expression.

= and <> can also be considered logical operators. However, unlike AND or OR they can’t be used to join more than two expressions.

Inputs AND OR = <> / !=
Input NOT


> 10 a = 10
> 20 b = 20
> 30 IF NOT a > b AND b = 20 OR a >= 5 THEN 60
> 40 PRINT "Test failed!"
> 50 STOP
> 60 PRINT "Test passed!"
Test passed!

Expressions can be inside brackets to change the order of evaluation. Compare the output when line 30 is changed:

> 30 IF NOT a > b AND (b = 20 OR a >= 5) THEN 60
Test failed!


The ON GOTO|GOSUB expr line1,line2,… statement will call a subroutine or branch to a line number in the list of line numbers corresponding to the ordinal
value of the evaluated expr. The first line number corresponds with an expr value of 1. expr must evaluate to an integer value.
If expr evaluates to less than 1 or greater than the number of provided line numbers execution continues on the next
statement without making a subroutine call or branch:

> 20 LET J = 2
> 30 ON J GOSUB 100,200,300
> 40 STOP
> 110 PRINT "J is ONE"
> 120 RETURN
> 210 PRINT "J is TWO"
> 220 RETURN
> 310 PRINT "J is THREE"
> 320 RETURN
J is TWO

It is also possible to call a subroutine depending upon the result of a conditional expression using the IFF function (see Ternary Functions below). In
the example below, if the expression evaluates to true, IFF returns a 1 and the subroutine is called, otherwise IFF returns a 0 and execution
continues to the next statement without making the call:

> 10 LET I = 10
> 20 LET J = 5
> 30 ON IFF (I > J, 1, 0) GOSUB 100
> 40 STOP
> 110 PRINT "I is greater than J"
> 120 RETURN
I is greater than J

Ternary Functions

As an alternative to branching, Ternary functions are provided.

  • IFF(x, y, z) - Evaluates x and returns y if true, otherwise returns z. y and z are expected to be numeric.
  • IF$(x, y$, z$) - As above, but y$ and z$ are expected to be strings.
> 10 LET I = 10
> 20 LET J = 5
> 30 PRINT IF$(I > J, "I is greater than J", "I is not greater than J")
> 40 K = IFF(I > J, 20, 30)
> 50 PRINT K
I is greater than J

User input

The INPUT statement is used to solicit input from the user (or read input from a file, see File I/O below):

> 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 semicolon, 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, excess commas will be left in place. Conversely, if not enough input values are
supplied, an error message will be printed and the user will be asked to re-input the values again.

Further, numeric input values must be valid numbers (integers or floating point).

> 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, and the user will be asked
to re-input the values again.

It is a limitation of this BASIC dialect that it is not possible to assign constants directly to array variables
within an INPUT statement, only simple variables.

File Input/Output

Data can be read from or written to files using the OPEN, FSEEK, INPUT, PRINT and CLOSE statements.

When a file is opened using the syntax OPENfilenameFOR INPUT|OUTPUT|APPEND AS #filenum [ELSE linenum] a
file number (#filenum) is assigned to the file, which if specified as the first argument of an INPUT or PRINT
statement, will direct the input or output to the file.

If there is an error opening a file and the optional ELSE option has been specified, program control
will branch to the specified line number, if the ELSE has not been provided an error message will be displayed.

If a file is opened for OUTPUT which does not exist, the file will be created, if the file does exist, its contents will
be erased and any new PRINT output will replace it. If a file is opened for APPEND an error will occur if the file
doesn’t exist (or the ELSE branch will occur if specified). If the file does exist, any PRINT statements will add to the end
of the file.

If an input prompt is specified on an INPUT statement being used for file I/O (i.e. #filenum is specified) an error
will be displayed.

The FSEEK #filenum,filepos statement will position the file pointer for the next INPUT statement.

The CLOSE #filenum statement will close the file.

> 20 PRINT #1,"0123456789Hello World!"
> 30 CLOSE #1
> 50 FSEEK #2,10
> 60 INPUT #2,A$
> 70 PRINT A$
Hello World!

Numeric functions

Selected numeric 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

  • INT(x) - Rounds down numbers to the lowest whole integer less than or equal to x

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

  • MAX(x, y[, z]…) - Returns the highest value from a list of expressions

  • MIN(x, y[, z]…) - Returns the lowest value from a list of expressions

> 10 PRINT MAX(-2, 0, 1.5, 4)
> 20 PRINT MIN(-2, 0, 1.5, 4)
> 4
> -2
  • PI - Returns the value of pi.

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

  • RND(mode) - Psuedorandom number generator. The behavior is different depending on the value passed. If the value is positive, the result will be a new random value between 0 and 1 (including 0 but not 1). If the value is negative, it will be rounded down to the nearest integer and used to reseed the random number generator. Pseudorandom sequences can be repeated by reseeding with the same number.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
> 20 PRINT RND(1)

Random integers can be generated by combining RND and INT: e.g.

> 10 PRINT INT(RND(1) * 6) + 1

Seeds may not produce the same result on another platform.

  • RNDINT(lo, hi) - Generates a pseudo random integer N, where lo <= N <= hi. Uses the same seed as above.

  • ROUND(x) - Rounds number to the nearest integer.

  • 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

String functions

Some functions are provided to help you manipulate strings. Functions that return a string
have a ‘$’ suffix like string variables.

NOTE For compatibility with older basic dialetcs, all string indexes are 1 based.

The functions are:

  • ASC(x$) - Returns the character code for x$. x$ is expected to be a single character.
    Note that despite the name, this function can return codes outside the ASCII range.

  • CHR$(x) - Returns the character specified by character code x.

  • INSTR(x$, y$[, start[, end]]) - Returns position of y$ inside x$, optionally start searching
    at position start and end at end. Returns 0 if no match found.

  • LEN(x$) - Returns the length of x$.

  • LOWER$(x$) - Returns a lower-case version of x$.

  • MID$(x$, y[, z]) - Returns part of x$ starting at position y. If z is provided, that number of characters is returned, if omitted the entire rest of the string is returned

  • LEFT$(x$, y) - Returns the left most y characters from string x$. If y * exceeds the length of x$, the entire string will be returned.

  • RIGHT$(x$, y) - Returns the right most y characters from string x$. If y * exceeds the length of x$, the entire string will be returned.

  • STR$(x) - Returns a string representation of numeric value x.

  • UPPER$(x$) - Returns an upper-case version of x$

  • VAL(x$) - Attempts to convert x$ to a numeric value. If x$ is not numeric, returns 0.

  • TAB(x) - When included in a PRINT statement print-list, specifies the position x on the line where the next text will be printed. If the specified position x is less than the current print position a newline is printed and the print location is set to the specified column. If the TAB function is used anywhere other than on a PRINT statement, it will return a string containing x spaces with no CR/LF

Examples for ASC, CHR$ and STR$

> 10 I = 65
> 20 J$ = CHR$(I) + " - " + STR$(I)
> 30 PRINT J$
> 40 PRINT ASC("Z")
A - 65

Strings may also be concatenated using the ‘+’ operator:

> 10 PRINT "Hello" + " there"
Hello there

Strings may be repeated using the ‘*’ operator:

> 10 PRINT "Hello " * 5
Hello Hello Hello Hello Hello

Example programs

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

  • 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.

  • PyBStartrek.bas - A port of the 1971 Star Trek text based strategy game.

  • adventure-fast.bas - A port of a 1979 text based Microsoft Adventure game.

  • bagels.bas - A guessing game, which made its first appearance in the book ‘BASIC Computer Games’ in 1978.

  • eliza.bas - A port of the early chatbot, posing as a therapist, originally created by Joseph Weizenbaum in 1964. This BASIC version can trace its lineage back to an implementation originally developed by Jeff Shrager in 1973.

  • oregon.bas - A port (of a port by the looks of it) of The Oregon Trail. This is a text based adventure game, originally developed by Don Rawitsch, Bill Heinemann, and Paul Dillenberger in 1971. This could still be a bit buggy, the listing I found wasn’t the greatest.

  • life.bas - An implementation of Conway’s Game of Life. This version is a port of the BASIC program which appeared in ‘BASIC Computer Games’ in 1978.

Informal grammar definition

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

ASC(string-expression) - Returns the character code of the result of string-expression.

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

CHR$(numerical-expression) - Returns the character specified by character code of the result of numerical-expression.

CLOSE #filenum - Closes an open file

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

FSEEK #filenum,filepos - Positions the file input pointer to the specified location within the open file, the next INPUT #filenum
will read starting at file position filepos

GOSUB line-number - Subroutine call

GOTO line-number - Unconditional branch

IF expression THEN line-number|basic-statement(s) [ELSE line-number|basic-statement(s)] - Conditional

IFF(expression, numeric-expression, numeric-expression) - Evaluates expression and returns the value of the result of the first numeric-expression if true, or the second if false.

IF$(expression, string-expression, string-expression) - Evaluates expression and returns the value of the result of the first string-expression if true, or the second if false.

INPUT [#filenum,|input-prompt;] simple-variable-list - Processes user or file input presented as a comma separated list

INSTR(hackstack-string-expression, needle-string-expression[, start-numeric-expression[, end-numeric-expression]]) - Returns position of first needle-string-expression inside first hackstack-string-expression, optionally start searching at position given by start-numeric-expression and optionally ending at position given by end-numeric-expression. Returns -1 if no match found.

LEFT$(string-expression, char-count) - Takes the result of string-expression and returns the left-most char-count characters. If char-count exceeds string length the entire string is returned.

LEN(string-expression) - Returns the length of the result of string-expression

[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

LOWER$(string-expression) - Returns a lower-case version of the result of string-expression.

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

MAX(expression-list) - Returns the highest value in expression-list

MID$(string-expression, start-position[, end-position]) - Takes the result of string-expression and returns part of it, starting at position start-position, and ending at end-position. end-position can
be omitted to get the rest of the string. If start-position or end-position are negative, the position is counted backwards from the end of the string.

MIN(expression-list) - Returns the lowest value in expression-list

ON expression GOSUB|GOTO line-number1,line-number2,… - Conditional subroutine call|branch - Program flow will be transferred either through a GOSUB subroutine call or a GOTO branch to the line number in the list of line numbers corresponding to the ordinal value of the evaluated expr. The first line number corresponds with an expr value of 1. expr must evaluate to an integer value.

OPENfilenameFOR INPUT|OUTPUT|APPEND AS #filenum [ELSE linenum] - Opens the specified file. Program control is transferred to linenum if an error occurs otherwise continues
on the next line.

PI - Returns the value of pi

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

PRINT [#filenum,]print-list - Prints a semicolon separated list of literals or variables to the screen or to a file. Included CR/LF by default, but this can be suppressed by ending the statement with a semicolon.

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

READ simple-variable-list - Reads a set of constants into the list of variables.

REM comment - Internal program documentation

RETURN - Return from a subroutine

RESTORE line-number - sets the line number that the next READ will start loading constants from. line-number must refer to a DATA statement

RIGHT$(string-expression, char-count) - Takes the result of string-expression and returns the right-most char-count characters. If char-count exceeds string length, the entire string is returned.

RND(mode) - For mode values >= 0 generates a pseudo random number N, where 0 <= N < 1. For values < 0 reseeds the PRNG

RNDINT(lo-numerical-expression, hi-numerical-expression) - Generates a pseudo random integer N, where lo-numerical-expression <= N <= hi-numerical-expression

ROUND(numerical-expression) - Rounds numerical-expression to the nearest integer

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

STR$(numerical-expression) - Returns a string representation of the result of numerical-expression

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

UPPER$(string-expression) - Returns an upper-case version of the result of string-expression

VAL(string-expression) - Attempts to convert the result of string-expression to a numeric value. If it is not numeric, returns 0.


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.
  • Decimal values less than one 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 or READ statement
  • Strings representing numbers (e.g. “10”) can actually be assigned to numeric variables in INPUT and READ statements without an
    error, Python will silently convert them to integers.


PyBasic is made available under the GNU General Public License, version 3.0 or later (GPL-3.0-or-later).