Getting Familiar
CircuitPython is a programming language based on Python, one of the fastest growing programming languages in the world. It is specifically designed to simplify experimenting and learning to code on low-cost microcontroller boards. Here are some guides which cover the basics:
- Welcome to CircuitPython!
- Adafruit Feather M4 Express (or whichever board you have chosen)
Be sure you have the latest CircuitPython loaded onto your board per the second guide.
CircuitPython is easiest to use within the Mu Editor. If you haven't previously used Mu, this guide will get you started.
The REPL code, itself, just needs the sys module (which is automatically pulled in in CircuitPython without an import statement). No additional libraries are required.
Loading the Code
Click the Download code.py link in the code listing below. Save the file to a place on your computer you'll remember (like a Downloads folder, etc.).
Plug in your Adafruit M4 Express board into your computer via a known good USB cable (with data and power lines). A flash drive named CIRCUITPY should appear.
If you see a flash drive named XXXXBOOT, reset the board via the reset button and see if the CIRCUITPY drive will appear. If not, you need to load the latest version of CircuitPython (4.0 or above) onto the board. See this guide for instructions then come back here.
Copy the file code.py from your download directory to the CIRCUITPY drive. Your code should start to run immediately.
"""
Scheme Interpreter in CircuitPython
Based on Lispy.py (c) Peter Norvig, 2010; See http://norvig.com/lispy2.html
Adafruit invests time and resources providing this open source code.
Please support Adafruit and open source hardware by purchasing
products from Adafruit!
Written by Dave Astels for Adafruit Industries
Copyright (c) 2019 Adafruit Industries
Licensed under the MIT license.
All text above must be included in any redistribution.
"""
# Initially we'll avoid all pylint's complaints.
# Over time we'll bring it in line.
# pylint: disable=wrong-import-order,no-member,missing-docstring,invalid-name
# pylint: disable=redefined-builtin,multiple-statements,too-many-branches
# pylint: disable=too-many-return-statements,no-else-return,bad-whitespace
# pylint: disable=superfluous-parens,exec-used,wrong-import-position
# pylint: disable=unnecessary-lambda,multiple-imports
# pylint: disable=misplaced-comparison-constant,too-few-public-methods
# pylint: disable=dangerous-default-value,unnecessary-semicolon
# pylint: disable=broad-except,bad-continuation
################ Symbol, Procedure, classes
import re, sys
from io import StringIO
import gc
class Symbol(str): pass
def Sym(s, symbol_table={}):
"""Find or create unique Symbol entry for str s in symbol table.
Returns the symbol.
:param s: the string form of the desired symbol
:param symbol_table: The symbol table (dictionary) to look up in, defaults to an empty dict.
"""
if s not in symbol_table:
symbol_table[s] = Symbol(s)
return symbol_table[s]
# Create some builtin sysmols
_quote, _if, _cond, _set, _define, _lambda, _begin, _definemacro, = map(Sym,
"quote if cond set! define lambda begin define-macro".split())
_quasiquote, _unquote, _unquotesplicing = map(Sym,
"quasiquote unquote unquote-splicing".split())
class Procedure(object):
"A user-defined Scheme procedure."
def __init__(self, parms, exp, env):
"""Create a procedure.
:param parms: parameter names
:param exp: The expression for the body of the procedure
:param env: The lexical environment to which the procedure belongs
"""
self.parms, self.exp, self.env = parms, exp, env
def __call__(self, *args):
"""Evaluate a procedure.
:param args: the arguments for the procedure evaluation
"""
return eval(self.exp, Env(self.parms, args, self.env))
################ parse, read, and user interaction
def parse(inport):
"""Parse a program: read and expand/error-check it.
:param inport: where to parse from
"""
# Backwards compatibility: given a str, convert it to an InPort
if isinstance(inport, str):
inport = InPort(StringIO(inport))
return expand(read(inport), toplevel=True)
eof_object = Symbol('#<eof-object>') # Note: uninterned; can't be read
class InPort(object):
"An input port. Retains a line of chars."
tokenizer = r""" *(,@|[('`,)]|"(?:\\.|[^\\"])*"|;.*|[^ ('"`,;)]*)(.*)"""
def __init__(self, afile):
"""Create a new InPort.
:param afile: the file-like object that characters will come from
"""
self._file = afile
self.line = ''
def next_token(self):
"""Return the next token, reading new text into line buffer if needed."""
while True:
if self.line == '':
self.line = self._file.readline()
if self.line == '':
return eof_object
self.line = self.line.strip()
m = re.match(InPort.tokenizer, self.line)
token = m.group(1)
self.line = m.group(2)
if token != '' and not token.startswith(';'):
return token
def readchar(inport):
"""Read and return the next character from an input port.
:param inport: Where to read from
"""
if inport.line != '':
ch, inport.line = inport.line[0], inport.line[1:]
return ch
else:
return inport.file.read(1) or eof_object
def read(inport):
"""Read a Scheme expression from an input port.
:param inport: where to read from
"""
def read_ahead(token):
if '(' == token:
L = []
while True:
token = inport.next_token()
if token == ')':
return L
else:
L.append(read_ahead(token))
elif ')' == token:
raise SyntaxError('unexpected )')
elif token in quotes:
return [quotes[token], read(inport)]
elif token is eof_object:
raise SyntaxError('unexpected EOF in list')
else:
return atom(token)
# body of read:
token1 = inport.next_token()
return eof_object if token1 is eof_object else read_ahead(token1)
quotes = {"'":_quote, "`":_quasiquote, ",":_unquote, ",@":_unquotesplicing}
def atom(token):
"""Convert a token to its corresponding atomic value.
Numbers become numbers; #t and #f are booleans; "..." string; otherwise Symbol.
:param token: the token to convert"""
if token == '#t':
return True
elif token == '#f':
return False
elif token[0] == '"':
return token[1:-1]#.decode('string_escape')
try:
return int(token)
except ValueError:
try:
return float(token)
except ValueError:
return Sym(token)
def to_string(x):
"""Convert a Python object back into a Lisp-readable string.
:param x: the object to convert"""
if x is True:
return "#t"
elif x is False:
return "#f"
elif isa(x, Symbol):
return str(x)
# elif isa(x, str):
# return '"%s"' % x.encode('string_escape').replace('"',r'\"')
elif isa(x, str):
return '"%s"' % x.replace('"',r'\"')
elif isa(x, list):
return '('+' '.join(map(to_string, x))+')'
else:
return str(x)
def load(filename):
"""Eval every expression from a file.
:param filename: the name of the file to load
"""
if not filename.endswith('.scm'):
filename = filename + '.scm'
inport = InPort(open(filename))
while True:
try:
x = parse(inport)
if x is eof_object: return
eval(x)
except Exception as e:
sys.print_exception(e)
############ REPL history support
history_max_size = 40
history = []
def add_to_history(line):
"""Add a line to the REPL history.
:param line: the line to be added
"""
global history
if line and (not history or history[0] != line):
history = history[:history_max_size - 1]
history.insert(0, line.strip())
def get_history(offset):
"""Retrieve a line from the history.
:param offset: The index into the history; 0 is the most recent and
larger offsets are older
"""
if offset < 0 or offset >= len(history):
return ''
return history[offset]
def repl():
"A read-eval-print loop with readline-like behavior."
input = ''
line = ''
index = 0
ctrl_c_seen = False
while True: # for each line
try:
if input:
prompt = '... '
else:
prompt = '==> '
sys.stdout.write(prompt)
index = 0
line = ''
history_offset = -1
while True: # for each character
ch = ord(sys.stdin.read(1))
# if ch == 3: # CTRL-C
# print('ctrl-c from ch == 3')
# if ctrl_c_seen:
# return
# ctrl_c_seen = True
# input = ''
# sys.stdout.write('\n')
# break
ctrl_c_seen = False
if 32 <= ch <= 126: # printable character
line = line[:index] + chr(ch) + line[index:]
index += 1
elif ch in {10, 13}: # EOL - try to process
if input:
input = input + ' ' + line.strip()
else:
input = line.strip()
add_to_history(line.strip())
line = ''
try:
x = parse(input)
if x is eof_object:
raise SyntaxError('unexpected EOF in list')
val = eval(x)
if val is not None:
sys.stdout.write('\n{0}'.format(to_string(val)))
input = ''
except SyntaxError as e:
if str(e) != 'unexpected EOF in list':
sys.stdout.write('\n')
sys.stdout.write(str(e))
input = ''
sys.stdout.write('\n')
break
#####################
elif ch == 1: # CTRL-A: start of line
index = 0
elif ch == 5: # CTRL-E: end of line
index = len(line)
#####################
elif ch == 2: # CTRL-B: back a word
while index > 0 and line[index-1] == ' ':
index -= 1
while index > 0 and line[index-1] != ' ':
index -= 1
elif ch == 6: # CTRL-F: forward a word
while index < len(line) and line[index] == ' ':
index += 1
while index < len(line) and line[index] != ' ':
index += 1
#####################
elif ch == 4: # CTRL-D: delete forward
if index < len(line):
line = line[:index] + line[index+1:]
elif ch == 11: # CTRL-K: clear to end of line
line = line[:index]
elif ch in {8, 127}: # backspace/DEL
if index > 0:
line = line[:index - 1] + line[index:]
index -= 1
#####################
elif ch == 20: # CTRL-T: transpose characters
if index > 0 and index < len(line):
ch1 = line[index - 1]
ch2 = line[index]
line = line[:index - 1] + ch2 + ch1 + line[index + 1:]
#####################
elif ch == 27: # ESC
next1, next2 = ord(sys.stdin.read(1)), ord(sys.stdin.read(1))
if next1 == 91: # [
if next2 == 68: # left arrow
if index > 0:
index -= 1
else:
sys.stdout.write('\x07')
elif next2 == 67: # right arrow
if index < len(line):
index += 1
else:
sys.stdout.write('\x07')
elif next2 == 66: # down arrow
if history_offset > -1:
history_offset -= 1
line = get_history(history_offset)
index = len(line)
else:
sys.stdout.write('\x07')
elif next2 == 65: # up arrow
if history_offset < len(history) - 1:
history_offset += 1
line = get_history(history_offset)
index = len(line)
else:
sys.stdout.write('\x07')
else:
print('Unknown character: {0}'.format(ch))
# Update screen
sys.stdout.write("\x1b[1000D") # Move all the way left
sys.stdout.write("\x1b[0K") # Clear the line
sys.stdout.write(prompt)
sys.stdout.write(line)
sys.stdout.write("\x1b[1000D") # Move all the way left again
sys.stdout.write("\x1b[{0}C".format(len(prompt) + index)) # Move cursor too index
# sys.stdout.flush()
except KeyboardInterrupt:
if ctrl_c_seen:
return
ctrl_c_seen = True
input = ''
sys.stdout.write('\n')
except Exception as e:
sys.stdout.write('\n')
sys.print_exception(e)
sys.stdout.write('\n')
input = ''
################ Environment class
class Env(object):
"An environment: a dict of {'var':val} pairs, with an outer Env."
def __init__(self, parms=(), args=(), outer=None):
# Bind parm list to corresponding args, or single parm to list of args
self.storage = {}
self.outer = outer
if isa(parms, Symbol):
self.storage.update({str(parms):list(args)})
else:
if len(args) != len(parms):
raise TypeError('expected %s, given %s, '
% (to_string(parms), to_string(args)))
try:
self.storage.update(zip([str(p) for p in parms],args))
except TypeError as e:
sys.print_exception(e)
def find(self, var):
"Find the innermost Env where var appears."
if str(var) in self.storage: return self
elif self.outer is None:
raise LookupError(str(var))
else: return self.outer.find(var)
def is_pair(x): return x != [] and isa(x, list)
def cons(x, y): return [x]+y
def callcc(proc):
"Call proc with current continuation; escape only"
ball = RuntimeWarning("Sorry, can't continue this continuation any longer.")
def throw(retval): ball.retval = retval; raise ball
try:
return proc(throw)
except RuntimeWarning as w:
if w is ball: return ball.retval
else: raise w
def mod_vars(mod):
names = [i for i in dir(mod) if i[0] != '_']
values = [getattr(mod, i) for i in names]
return dict(zip(names, values))
def add_globals(self):
"Add some Scheme standard procedures."
import math, operator as op
self.storage.update(mod_vars(math))
# self.update(mod_vars(cmath))
self.storage.update({
'+':op.add, '-':op.sub, '*':op.mul, '/':op.truediv, 'not':op.not_,
'>':op.gt, '<':op.lt, '>=':op.ge, '<=':op.le, '=':op.eq,
'equal?':op.eq, 'eq?':op.is_, 'length':len, 'cons':cons,
'car':lambda x:x[0], 'cdr':lambda x:x[1:], 'append':op.add,
'list':lambda *x:list(x), 'list?': lambda x:isa(x,list),
'null?':lambda x:x==[], 'symbol?':lambda x: isa(x, Symbol),
'boolean?':lambda x: isa(x, bool), 'pair?':is_pair,
'port?': lambda x:isa(x,file), 'apply':lambda proc,l: proc(*l),
'eval':lambda x: eval(expand(x)), 'load':lambda fn: load(fn), 'call/cc':callcc,
'open-input-file':open,'close-input-port':lambda p: p.file.close(),
'open-output-file':lambda f:open(f,'w'), 'close-output-port':lambda p: p.close(),
'eof-object?':lambda x:x is eof_object, 'read-char':readchar,
'read':read, 'write':lambda x,port=sys.stdout:port.write(to_string(x)),
'display':lambda x,port=sys.stdout:port.write(x if isa(x,str) else to_string(x)),
'newline':lambda port=sys.stdout:port.write("\n")})
return self
isa = isinstance
global_env = add_globals(Env())
################ eval (tail recursive)
def eval(x, env=global_env):
"Evaluate an expression in an environment."
while True:
if isa(x, Symbol): # variable reference
return env.find(str(x)).storage[str(x)]
elif not isa(x, list): # constant literal
return x
elif x[0] is _quote: # (quote exp)
(_, exp) = x
return exp
elif x[0] is _if: # (if test conseq alt)
(_, test, conseq, alt) = x
x = (conseq if eval(test, env) else alt)
elif x[0] is _cond: # (cond (test code)...)
for clause in x[1:]:
if eval(clause[0], env):
for exp in clause[1:-1]:
eval(exp, env)
x = clause[-1]
break
elif x[0] is _set: # (set! var exp)
(_, var, exp) = x
env.find(var).storage[str(var)] = eval(exp, env)
return None
elif x[0] is _define: # (define var exp)
(_, var, exp) = x
env.storage[str(var)] = eval(exp, env)
return None
elif x[0] is _lambda: # (lambda (var*) exp)
(_, vars, exp) = x
return Procedure(vars, exp, env)
elif x[0] is _begin: # (begin exp+)
for exp in x[1:-1]:
eval(exp, env)
x = x[-1]
else: # (proc exp*)
exps = [eval(exp, env) for exp in x]
proc = exps.pop(0)
if isa(proc, Procedure):
x = proc.exp
env = Env(proc.parms, exps, proc.env)
else:
return proc(*exps)
################ expand
def expand(x, toplevel=False):
"Walk tree of x, making optimizations/fixes, and signaling SyntaxError."
require(x, x!=[], "Empty list can't be expanded") # () => Error
if not isa(x, list): # constant => unchanged
return x
elif x[0] is _quote: # (quote exp)
require(x, len(x)==2)
return x
elif x[0] is _if:
if len(x)==3: x = x + [None] # (if t c) => (if t c None)
require(x, len(x)==4)
return list(map(expand, x))
elif x[0] is _cond:
require(x, len(x) > 1)
for clause in x[1:]:
require (clause, len(clause) >= 2)
return list(map(expand, x))
elif x[0] is _set:
require(x, len(x)==3);
var = x[1] # (set! non-var exp) => Error
require(x, isa(var, Symbol), "can set! only a symbol")
return [_set, var, expand(x[2])]
elif x[0] is _define or x[0] is _definemacro:
require(x, len(x)>=3)
_def, v, body = x[0], x[1], x[2:]
if isa(v, list) and v: # (define (f args) body)
f, args = v[0], v[1:] # => (define f (lambda (args) body))
return expand([_def, f, [_lambda, args]+body])
else:
require(x, len(x)==3) # (define non-var/list exp) => Error
require(x, isa(v, Symbol), "can define only a symbol")
exp = expand(x[2])
if _def is _definemacro:
require(x, toplevel, "define-macro only allowed at top level")
proc = eval(exp)
require(x, callable(proc), "macro must be a procedure")
macro_table[v] = proc # (define-macro v proc)
return None # => None; add v:proc to macro_table
return [_define, v, exp]
elif x[0] is _begin:
if len(x)==1: return None # (begin) => None
else: return [expand(xi, toplevel) for xi in x]
elif x[0] is _lambda: # (lambda (x) e1 e2)
require(x, len(x)>=3) # => (lambda (x) (begin e1 e2))
vars, body = x[1], x[2:]
require(x, (isa(vars, list) and all(isa(v, Symbol) for v in vars))
or isa(vars, Symbol), "illegal lambda argument list")
exp = body[0] if len(body) == 1 else [_begin] + body
return [_lambda, vars, expand(exp)]
elif x[0] is _quasiquote: # `x => expand_quasiquote(x)
require(x, len(x)==2)
return expand_quasiquote(x[1])
elif isa(x[0], Symbol) and x[0] in macro_table:
return expand(macro_table[x[0]](*x[1:]), toplevel) # (m arg...)
else: # => macroexpand if m isa macro
return list(map(expand, x)) # (f arg...) => expand each
def require(x, predicate, msg="wrong length"):
"Signal a syntax error if predicate is false."
if not predicate: raise SyntaxError(to_string(x)+': '+msg)
_append, _cons, _let = map(Sym, "append cons let".split())
def expand_quasiquote(x):
"""Expand `x => 'x; `,x => x; `(,@x y) => (append x y) """
if not is_pair(x):
return [_quote, x]
require(x, x[0] is not _unquotesplicing, "can't splice here")
if x[0] is _unquote:
require(x, len(x)==2)
return x[1]
elif is_pair(x[0]) and x[0][0] is _unquotesplicing:
require(x[0], len(x[0])==2)
return [_append, x[0][1], expand_quasiquote(x[1:])]
else:
return [_cons, expand_quasiquote(x[0]), expand_quasiquote(x[1:])]
def let(*args):
args = list(args)
x = cons(_let, args)
require(x, len(args)>1)
bindings, body = args[0], args[1:]
require(x, all(isa(b, list) and len(b)==2 and isa(b[0], Symbol)
for b in bindings), "illegal binding list")
vars, vals = zip(*bindings)
return [[_lambda, list(vars)]+list(map(expand, body))] + list(map(expand, vals))
macro_table = {_let:let} ## More macros can go here
################ core builtins
eval(parse("""(begin
(define-macro and (lambda args
(if (null? args) #t
(if (= (length args) 1) (car args)
`(if ,(car args) (and ,@(cdr args)) #f)))))
(define-macro or (lambda args
(if (null? args) #f
(if (= (length args) 1) (car args)
`(if (not ,(car args)) (or ,@(cdr args)) #t)))))
(define-macro when (lambda args
`(if ,(car args) (begin ,@(cdr args)))))
(define-macro unless (lambda args
`(if (not ,(car args)) (begin ,@(cdr args)))))
;; More macros can also go here
)"""))
################ hardware builtins
import time
import board
import busio
from adafruit_bus_device.i2c_device import I2CDevice
import digitalio
import analogio
board_pins = mod_vars(board)
def get_board_pins():
return list(board_pins.keys())
def get_pin(pin_name):
try:
return board_pins[pin_name]
except KeyError:
print('{0} is not a valid pin name'.format(pin_name))
return None
def make_digital_pin(pin, direction, pull=None):
p = digitalio.DigitalInOut(pin)
p.direction = direction
if direction is digitalio.Direction.INPUT:
p.pull = pull
return p
def make_analog_pin(pin, direction):
if direction is digitalio.Direction.INPUT:
p = analogio.AnalogIn(pin)
else:
p = analogio.AnalogOut(pin)
return p
def set_pin_value(pin, value):
pin.value = value
def get_pin_value(pin):
return pin.value
def i2c_bus(scl, sda):
return busio.I2C(scl, sda)
def i2c_device(i2c, address):
return I2CDevice(i2c, address)
def execfile(f):
exec(open(f).read())
def load_device(device_driver_name):
try:
execfile('./devices/{0}.py'.format(device_driver_name))
except OSError:
pass
try:
load('./devices/{0}.scm'.format(device_driver_name))
except OSError:
pass
global_env.storage.update({
'load-device':load_device,
'board-pins':get_board_pins,
'board':get_pin,
'digital-pin':make_digital_pin,
'analog-pin':make_analog_pin,
'**INPUT**':digitalio.Direction.INPUT,
'**OUTPUT**':digitalio.Direction.OUTPUT,
'**PULLUP**':digitalio.Pull.UP,
'**PULLDOWN**':digitalio.Pull.DOWN,
'pin-value':get_pin_value,
'pin-value!':set_pin_value,
'i2c':i2c_bus,
'sleep':time.sleep
})
if __name__ == '__main__':
print('CircuitScheme version 1.0: {0} bytes free.\n'.format(gc.mem_free()))
try:
load("code")
except OSError:
pass
repl()
Code Walkthrough
The full CircuitScheme code is above. The following is a walk through the REPL code piece by piece.
Command History
These two functions handle the in-memory history list. As such, it doesn't persist between restarts. This is due to limitations on CircuitPython has in currently writing to the filesystem. This may be addressed in a future version.
add_to_history takes a line and adds it to the history if it's not an empty line and it's not the same as the most recent line added to the history. There's a cap on how many lines are kept. The code below sets that to 40. Adding to the history means inserting the new line at index 0. This makes accessing the history easier.
get_history takes an index that increases as you go further back. 0 is the most recent line added.
history_max_size = 40
history = []
def add_to_history(line):
global history
if line and (not history or history[0] != line):
history = history[:history_max_size - 1]
history.insert(0, line.strip())
def get_history(offset):
if offset < 0 or offset >= len(history):
return ''
return history[offset]
The REPL
After setting some variables, we enter a while True: loop. The general idea with a REPL is that it normally never exits: getting and processing one command after another. The only way out of this loop is to press Ctrl-c twice in a row.
At the moment, exiting isn't as smooth as it should be. For reasons TBD, another character is required after each Ctrl-c for them to be handled.
To manage the Ctrl-c handling, we have a flag that notes when a Ctrl-c is seen. It gets reset after a non Ctrl-c is read. If another Ctrl-c is encountered before that happens (as with two Ctrl-c pressed in a row) then the loop is exited. This is handled at the end of the loop (see below).
This outer loop runs once for each line of input. The first thing done is generate the prompt. User input is accumulated, line by line, in the variable input. At the start of an entry (a Lisp expression in this case). input is empty. If a partial expression has been entered (i.e. with more remaining to be entered on subsequent lines) input will be non-empty. Thus, we can use that information to choose between an initial or continuation prompt.
We set the cursor to be at the start of the line, the line to be empty, and the current history index to -1, i.e. not pointing into the history buffer.
ctrl_c_seen = False
while True: # for each line
try:
if input:
prompt = '... '
else:
prompt = '==> '
sys.stdout.write(prompt)
index = 0
line = ''
history_offset = -1
# the guts of the REPL go here
except KeyboardInterrupt:
print('ctrl-c from KeyboardInterrupt')
if ctrl_c_seen:
return
ctrl_c_seen = True
input = ''
sys.stdout.write('\n')
except Exception as e:
sys.stdout.write('\n')
sys.print_exception(e)
sys.stdout.write('\n')
input = ''
That comment about the guts of the REPL is where there's a loop that iterates for each character that gets entered. Logically it starts by reading a single character and clearing the ctrl_c_seen flag.
while True: # for each character
ch = ord(sys.stdin.read(1))
ctrl_c_seen = False
The bulk of the loop body is a multi-branch conditional that looks at that character and decides what to do.
First it checks for a printable character that should be added to the line.
if 32 <= ch <= 126: # printable character
line = line[:index] + chr(ch) + line[index:]
index += 1
We can't just append the character to line since (as we'll see) cursor movement within the line is supported. That means that the cursor could be anywhere in the line, so the character has to be inserted in the appropriate place. That's where the index variable comes in. That's where the cursor is, and that's where characters get inserted into the line. Once that happens, index is incremented to account for the new character.
The next type of character checked for are the line terminators: either a carriage return (13) or a linefeed (10).
In this case, the line accumulated is appended to input (with an interstitial space) and added to the history. Then we try to parse the input. If the expression is incomplete either a syntax error is raised, or an EOF sentinel is returned. In either case a new line is started. Any other exception will be printed and the accumulated input discarded.
If the input does parse, the result is evaluated. If that results in a non-None value, it is printed to the console.
elif ch in {10, 13}: # EOL - try to process
if input:
input = input + ' ' + line.strip()
else:
input = line.strip()
add_to_history(line.strip())
line = ''
try:
x = parse(input)
if x is eof_object:
raise SyntaxError('unexpected EOF in list')
val = eval(x)
if val is not None:
sys.stdout.write('\n{0}'.format(to_string(val)))
input = ''
except SyntaxError as e:
if str(e) != 'unexpected EOF in list':
sys.stdout.write('\n')
sys.stdout.write(str(e))
input = ''
sys.stdout.write('\n')
break
Next up are some single-character cursor movement keys:
Control-a - Start of line
Control-e - End of line
Control-b - back/left a word
Control-f - forward/right a word
elif ch == 1: # CTRL-A: start of line
index = 0
elif ch == 5: # CTRL-E: end of line
index = len(line)
elif ch == 2: # CTRL-B: back a word
while index > 0 and line[index-1] == ' ':
index -= 1
while index > 0 and line[index-1] != ' ':
index -= 1
elif ch == 6: # CTRL-F: forward a word
while index < len(line) and line[index] == ' ':
index += 1
while index < len(line) and line[index] != ' ':
index += 1
Three deletion options are supported:
Control-d - delete the character the cursor is on, aka forward delete
Control-k - delete the line from the cursor to the end of the line
backspace/delete - delete the character to the left of the cursor
elif ch == 4: # CTRL-D: delete forward
if index < len(line):
line = line[:index] + line[index+1:]
elif ch == 11: # CTRL-K: clear to end of line
line = line[:index]
elif ch in {08, 127}: # backspace/DEL
if index > 0:
line = line[:index - 1] + line[index:]
index -= 1
The next one is useful. As with the others, this is from Emacs. Control-t switches the character under the cursor with the one to the left of it. The cursor is left where it was.
elif ch == 20: # CTRL-T: transpose characters
if index > 0 and index < len(line):
ch1 = line[index - 1]
ch2 = line[index]
line = line[:index - 1] + ch2 + ch1 + line[index + 1:]
Next we have the four direction arrows: up, down, left, and right. These are not single keys like the previous commands. Instead, they use the VT100/ANSI key code sequences.
These sequences all begin with an ESC character followed by an opening square brace. I.e. codes 27 and 91. Following that opening sequence the next keycode indicates which arrow:
68 - left
67 - right
66 - down
65 - up
Left and right movement is limited by the start and end of the line. If you try to move past those boundaries, the terminal bell is sounded by writing the BELL character (07 or Control-g) to the console.
Up and down travel through the history: back and forward, respectively. Trying to move outside the accumulated history will also ring the bell and not change anything. If there is a history entry to move to, the index is updated and that line fetched form the history and used to replace the line being edited.
elif ch == 27: # ESC
next1, next2 = ord(sys.stdin.read(1)), ord(sys.stdin.read(1))
if next1 == 91: # [
if next2 == 68: # left arrow
if index > 0:
index -= 1
else:
sys.stdout.write('\x07')
elif next2 == 67: # right arrow
if index < len(line):
index += 1
else:
sys.stdout.write('\x07')
elif next2 == 66: # down arrow
if history_offset > -1:
history_offset -= 1
line = get_history(history_offset)
index = len(line)
else:
sys.stdout.write('\x07')
elif next2 == 65: # up arrow
if history_offset < len(history) - 1:
history_offset += 1
line = get_history(history_offset)
index = len(line)
else:
sys.stdout.write('\x07')
Finally there's a catch-all case for anything else.
else:
print('Unknown character: {0}'.format(ch))
After the character has been handled, the line is updated. This makes heavy use of the VT100/ANSI control sequences to:
- Move all the way to the beginning of the line,
- clear the line,
- output the prompt,
- output the line,
- move back to the beginning of the line, and
- move right past the prompt, and
indexcharacters more.
To be sure we get all the way left, we tell the terminal to move 1000 characters left. It ignores any excess.
sys.stdout.write("\x1b[1000D") # Move all the way left
sys.stdout.write("\x1b[0K") # Clear the line
sys.stdout.write(prompt)
sys.stdout.write(line)
sys.stdout.write("\x1b[1000D") # Move all the way left again
sys.stdout.write("\x1b[{0}C".format(len(prompt) + index)) # Move cursor too index
Adapting to your needs
There are two functions that tie the REPL into CircuitScheme, making it a REPL rather than simply a line editor.
The first is the call to parse which attempts to parse input. If it can, then input contains a legal CircuitScheme expression. If not, then input contains a partial expression, and more is needed so we keep looping to get further lines. This continues until input can be parsed. You can replace the call to parse with a call to some function that checks if input is usable as is, or if more is required. Note that parse does this either by raising a SyntaxError with a specific message, or returning an EOF sentinel. That will need tweaking in your case.
The other function is eval, which is called on the result of parse. Replace this with a call to a function that does whatever is required with your input. This will most likely be a command processor of some sort.
Page last edited January 20, 2025
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