Checking function calls

[Fredrik Tolf]

If I have a variable which points to a function, can I check if certain
argument list matches what the function wants before or when calling it?

Currently, I'm trying to catch a TypeError when calling the function
(since that is what is raised when trying to call it with an illegal
list), but that has the rather undesirable side effect of also catching
any TypeErrors raised inside the function. Is there a way to avoid that?

Fredrik Tolf

[Ravi Teja]

You can match if the list contains the legal number of arguments with.
func_variable.func_code.co_argcount

Type checking arguments has to be done manually since Python is a
dynamic language. Perhaps, you could try some typechecking decorators.
http://www.ilowe.net/software/typecheck/

From the docs, this one raises TypeCheckError (not TypeError), so you

should do fine.

[James Stroud]

Fredrik Tolf wrote:

If I have a variable which points to a function, can I check if certain
argument list matches what the function wants before or when calling it?

Currently, I'm trying to catch a TypeError when calling the function
(since that is what is raised when trying to call it with an illegal
list), but that has the rather undesirable side effect of also catching
any TypeErrors raised inside the function. Is there a way to avoid that?

Fredrik Tolf

Since python is "weakly typed", you will not be able to check what
"type" of arguments a function expects. TypeErrors relating to the type
of argemtns you pass will be raised inside the function only and not
when it is called. I.e. there is no type checking when a function is
called, only in its body.

Types erros can be raised when calling a function like this:

func(*avar)

or

func(**avar)

In the former case, if avar is not a sequence, you will get a TypeError.
In the latter case, if avar is not a dictionary, you will get a TypeError.

In this latter case, if your dictionary has keys that are not in the
parameter list, you will get a TypeError.

This would be similar to explicitly specifying a keyword argument that
is not in the parameter list (e.g. func(aparam=someval)).

TypeError will be raised also if you give an improper number of
arguments to the function. So, if you want to know the number of
arguments expected, I've found this works:

py> def func(a,b,c):
.... print a,b,c
....
py> func.func_code.co_argcount
3

To check the parameter names, use

py> func.func_code.co_names
('a', 'b', 'c')
James

--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/

[bruno at modulix]

James Stroud wrote:
(snip)

Since python is "weakly typed",

s/weakly/dynamically/

(snip)

--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'o****@xiludom.gro'.split('@')])"

[Roy Smith]

Fredrik Tolf <py*********@python.org> wrote:

If I have a variable which points to a function, can I check if certain
argument list matches what the function wants before or when calling it?

Currently, I'm trying to catch a TypeError when calling the function
(since that is what is raised when trying to call it with an illegal
list), but that has the rather undesirable side effect of also catching
any TypeErrors raised inside the function. Is there a way to avoid that?

The only way is to read the documentation for the function (or, the
source code).

Can you be a little more specific about what you're trying to do? Can
you post your code?

[gene tani]

Roy Smith wrote:

Fredrik Tolf <py*********@python.org> wrote:

If I have a variable which points to a function, can I check if certain
argument list matches what the function wants before or when calling it?

Currently, I'm trying to catch a TypeError when calling the function
(since that is what is raised when trying to call it with an illegal
list), but that has the rather undesirable side effect of also catching
any TypeErrors raised inside the function. Is there a way to avoid that?

The only way is to read the documentation for the function (or, the
source code).

Can you be a little more specific about what you're trying to do? Can
you post your code?

if you know ahead of runtime which methods are at issue and what method
signatures you want, multimethod decorators, maybe?
http://www.artima.com/weblogs/viewpo...?thread=101605
(read comments: PJE, ian bicking

or you can arg-test inside the method, which isn't much fun
http://www.python.org/doc/faq/progra...hods-in-python

[Joel Hedlund]

If you have access to the source of the function you want to call (and know what kind of data types it wants in its args) you can raise something else for bad parameters, eg:

----------------------------------------------------------------
class ArgTypeError(TypeError):
def __init__(self, arg):
self.arg = arg

def __str__(self):
return "bad type for argument %r" % self.arg

def moo(cow):
if not isinstance(cow, str):
raise ArgTypeError('cow')
print "%s says moo!" % cow
# this error is not caused by wrong arg type:
var = 1 + 's'

function = moo

for arg in [1, 'rose']:
try:
function(arg)
except ArgTypeError, e:
print e
----------------------------------------------------------------

Output:
----------------------------------------------------------------
bad type for argument 'cow'
rose says moo!

Traceback (most recent call last):
File "/merlot1/yohell/eraseme/test.py", line 23, in -toplevel-
make_noise('rose')
File "/merlot1/yohell/eraseme/test.py", line 13, in moo
raise TypeError
TypeError
----------------------------------------------------------------

Hope it helps
/Joel Hedlund

[Fredrik Tolf]

On Mon, 2006-03-06 at 20:25 -0800, James Stroud wrote:

Fredrik Tolf wrote:

If I have a variable which points to a function, can I check if certain
argument list matches what the function wants before or when calling it?

Currently, I'm trying to catch a TypeError when calling the function
(since that is what is raised when trying to call it with an illegal
list), but that has the rather undesirable side effect of also catching
any TypeErrors raised inside the function. Is there a way to avoid that?

Fredrik Tolf
Since python is "weakly typed", you will not be able to check what
"type" of arguments a function expects. [...]

Sorry, it seems I made myself misunderstood. I don't intend to check the
type of the values that I pass to a function (I'm well aware that Python
is dynamically typed). The reason I refer to TypeError is because that
seems to be the exception raised by Python when I attempt to call a
function with an argument list that wouldn't be valid for it (for
example due to the number of parameters being wrong). I just want to
check in advance whether a certain arglist would be valid to call a
certain function.
So, if you want to know the number of
arguments expected, I've found this works:

py> def func(a,b,c):
... print a,b,c
...
py> func.func_code.co_argcount
3

Not very well, though:

def a(b, c, *d): pass .... print a.func_code.co_argcount

2

Here, that would indicate that I could only call `a' with two arguments,
while in fact I could call it with two or more arguments.

More exactly, what I'm trying to do is this; I writing a small protocol
server class as part of a program, and I thought it would be convenient
to be able to define new commands in the protocol by just adding a
function for them, like this:

class client:
# ...
# Read is called from the event driver when data is available on the
# socket.
def read(self):
self.buf += self.sk.recv(65536)
for c in self.buf.split("\n")[:-1]:
cv = tokenize(c)
if len(cv) > 0:
f = getattr(self, "cmd_" + cv[0], None)
if callable(f):
f(*cv[1:])
else:
self.reply("err", "unk")
# Example:
def cmd_echo(self, arg1):
self.reply("ok", arg1)

So basically, I want to be able to handle to client giving the wrong
number of arguments and reply with an error rather than the server
process crashing and dying. Therefore, I'm currently catching TypeErrors
when calling the function:

# ...
if callable(f):
try:
f(*cv[1:])
except TypeError:
self.reply("err", "argno")
# ...

However, this results in bugs in the server code that would cause
TypeError to reply to the client that it had the wrong number of args,
and I can't see what the real cause is without turning off that check
while debugging. Therefore, I'd like to check in advance whether cv[1:]
would raise a TypeError upon calling f with it or not. And of course,
I'd like to be able to use cmd_* functions with variable arglists as
well.

Surely, there ought to be some way in python to see whether a certain
argument list would be valid for a function call, without checking magic
values of f.func_code?

Fredrik Tolf

[Peter Otten]

Fredrik Tolf wrote:

# ...
if callable(f):
****try:
********f(*cv[1:])
****except*TypeError:
********self.reply("err",*"argno")
# ...

However, this results in bugs in the server code that would cause
TypeError to reply to the client that it had the wrong number of args,
and I can't see what the real cause is without turning off that check
while debugging. Therefore, I'd like to check in advance whether cv[1:]
would raise a TypeError upon calling f with it or not. And of course,
I'd like to be able to use cmd_* functions with variable arglists as
well.

If you must, just check the error message, not just the type of exception.

I'd just drop the cryptical error codes like 'argno' and 'unk' and pass on
the actual error message:

try:
f(*cv[1:])
except:
tp, exc, traceback = sys.exc_info()
self.reply("err", str(exc))

Note that the check whether f is callable is also gone.

Peter

[James Stroud]

Fredrik Tolf wrote:

On Mon, 2006-03-06 at 20:25 -0800, James Stroud wrote:

Fredrik Tolf wrote:

If I have a variable which points to a function, can I check if certain
argument list matches what the function wants before or when calling it?

Currently, I'm trying to catch a TypeError when calling the function
(since that is what is raised when trying to call it with an illegal
list), but that has the rather undesirable side effect of also catching
any TypeErrors raised inside the function. Is there a way to avoid that?

Fredrik Tolf

Since python is "weakly typed", you will not be able to check what
"type" of arguments a function expects. [...]

Sorry, it seems I made myself misunderstood. I don't intend to check the
type of the values that I pass to a function (I'm well aware that Python
is dynamically typed). The reason I refer to TypeError is because that
seems to be the exception raised by Python when I attempt to call a
function with an argument list that wouldn't be valid for it (for
example due to the number of parameters being wrong). I just want to
check in advance whether a certain arglist would be valid to call a
certain function.

So, if you want to know the number of
arguments expected, I've found this works:

py> def func(a,b,c):
... print a,b,c
...
py> func.func_code.co_argcount
3

Not very well, though:

def a(b, c, *d): pass
...
print a.func_code.co_argcount

2

Here, that would indicate that I could only call `a' with two arguments,
while in fact I could call it with two or more arguments.

More exactly, what I'm trying to do is this; I writing a small protocol
server class as part of a program, and I thought it would be convenient
to be able to define new commands in the protocol by just adding a
function for them, like this:

class client:
# ...
# Read is called from the event driver when data is available on the
# socket.
def read(self):
self.buf += self.sk.recv(65536)
for c in self.buf.split("\n")[:-1]:
cv = tokenize(c)
if len(cv) > 0:
f = getattr(self, "cmd_" + cv[0], None)
if callable(f):
f(*cv[1:])
else:
self.reply("err", "unk")
# Example:
def cmd_echo(self, arg1):
self.reply("ok", arg1)

So basically, I want to be able to handle to client giving the wrong
number of arguments and reply with an error rather than the server
process crashing and dying. Therefore, I'm currently catching TypeErrors
when calling the function:

# ...
if callable(f):
try:
f(*cv[1:])
except TypeError:
self.reply("err", "argno")
# ...

It might be more reasonable to program your cmd_* functions to be more
flexible themselves. This would be more pythonic. E.g.

def cmd_echo(self, *args):
if not len(args):
answer = 'OMITTED' # or raise your own exception
else:
answer = args[0]
self.reply("ok", answer)

Now, you never have to worry about length of the argument list.
You could, of course factor this behavior. Here's how I might do it:

# first make a custom exception class, called whatever you want
class ServerError(exception): pass

#... somewhere in your client class
def preprocess_args(self, args, numexpected):
"""
This is the more flexible way.
A less flexible way would to check for the exact number of
arguments. But that goes against the "lenient in what you accept"
maxim.
"""
if len(args) < numexpected:
raise ServerError, 'Too few args!'
else:
return args[:numexpected]

def cmd_echo(self, *args):
(answer,) = preprocess_args(args, 1) # preprocess returns tuple
self.reply("ok", answer)

#... elsewhere in your client class
if callable(f):
try:
f(*cv[1:])
except ServerError, e:
self.reply("err", "argno") # can also make use of exception e
You could factor even further with a dictionary:

minargs = {'echo' : 1, 'etc' : 5000}

#... somewhere in your client class
def preprocess_args(self, cv):
"""
This is the more flexible way.
A less flexible way would to check for the exact number of
arguments. But that goes against the "lenient in what you accept"
maxim.
"""
args = cv[1:]
expected = minargs[cv[0]]
if len(args) < expected:
raise ServerError, 'Too few args!'
else:
return args[:expected]

# ...
def cmd_echo(self, answer):
self.reply("ok", answer)

# ... elsewhere
if callable(f):
try:
args = preprocess_args(cv)
except ServerError, e:
self.reply("err", "argno") # can also make use of exception e
f(*args)

Now you shouldn't have to rewrite any cmd_* functions.

James

--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/

[James Stroud]

Fredrik Tolf wrote:

On Mon, 2006-03-06 at 20:25 -0800, James Stroud wrote:

Fredrik Tolf wrote:

If I have a variable which points to a function, can I check if certain
argument list matches what the function wants before or when calling it?

Currently, I'm trying to catch a TypeError when calling the function
(since that is what is raised when trying to call it with an illegal
list), but that has the rather undesirable side effect of also catching
any TypeErrors raised inside the function. Is there a way to avoid that?

Fredrik Tolf

Since python is "weakly typed", you will not be able to check what
"type" of arguments a function expects. [...]

Sorry, it seems I made myself misunderstood. I don't intend to check the
type of the values that I pass to a function (I'm well aware that Python
is dynamically typed). The reason I refer to TypeError is because that
seems to be the exception raised by Python when I attempt to call a
function with an argument list that wouldn't be valid for it (for
example due to the number of parameters being wrong). I just want to
check in advance whether a certain arglist would be valid to call a
certain function.

So, if you want to know the number of
arguments expected, I've found this works:

py> def func(a,b,c):
... print a,b,c
...
py> func.func_code.co_argcount
3

Not very well, though:

def a(b, c, *d): pass
...
print a.func_code.co_argcount

2

Here, that would indicate that I could only call `a' with two arguments,
while in fact I could call it with two or more arguments.

What is your goal? To avoid TypeError? In that case the minimum for no
error is 2, as *d could be empty. It would then be safe to check against
co_argcount to avoid errors:

py> def a(a,b,*c):
.... print a,b
....
py> a(*range(a.func_code.co_argcount))
0 1
py> a(*range(a.func_code.co_argcount + 5))
0 1
py> a(*range(a.func_code.co_argcount - 1))
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: a() takes at least 2 arguments (1 given)

If the function requires 3 arguments to work, you may want to change the
definition to reflect that fact as the *args are implicitly optional.
This will help with using co_argcount as a test.
James
--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/