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How to know the name of a classs loade like parameter on other class - Pyhton [duplicate]

This question already has answers here:
Getting the name of a variable as a string
(32 answers)
Closed 4 months ago.
Is it possible to get the original variable name of a variable passed to a function? E.g.
foobar = "foo"
def func(var):
print var.origname
So that:
func(foobar)
Returns:
>>foobar
EDIT:
All I was trying to do was make a function like:
def log(soup):
f = open(varname+'.html', 'w')
print >>f, soup.prettify()
f.close()
.. and have the function generate the filename from the name of the variable passed to it.
I suppose if it's not possible I'll just have to pass the variable and the variable's name as a string each time.

EDIT: To make it clear, I don't recommend using this AT ALL, it will break, it's a mess, it won't help you in any way, but it's doable for entertainment/education purposes.
You can hack around with the inspect module, I don't recommend that, but you can do it...
import inspect
def foo(a, f, b):
frame = inspect.currentframe()
frame = inspect.getouterframes(frame)[1]
string = inspect.getframeinfo(frame[0]).code_context[0].strip()
args = string[string.find('(') + 1:-1].split(',')
names = []
for i in args:
if i.find('=') != -1:
names.append(i.split('=')[1].strip())
else:
names.append(i)
print names
def main():
e = 1
c = 2
foo(e, 1000, b = c)
main()
Output:
['e', '1000', 'c']

To add to Michael Mrozek's answer, you can extract the exact parameters versus the full code by:
import re
import traceback
def func(var):
stack = traceback.extract_stack()
filename, lineno, function_name, code = stack[-2]
vars_name = re.compile(r'\((.*?)\).*$').search(code).groups()[0]
print vars_name
return
foobar = "foo"
func(foobar)
# PRINTS: foobar

Looks like Ivo beat me to inspect, but here's another implementation:
import inspect
def varName(var):
lcls = inspect.stack()[2][0].f_locals
for name in lcls:
if id(var) == id(lcls[name]):
return name
return None
def foo(x=None):
lcl='not me'
return varName(x)
def bar():
lcl = 'hi'
return foo(lcl)
bar()
# 'lcl'
Of course, it can be fooled:
def baz():
lcl = 'hi'
x='hi'
return foo(lcl)
baz()
# 'x'
Moral: don't do it.

Another way you can try if you know what the calling code will look like is to use traceback:
def func(var):
stack = traceback.extract_stack()
filename, lineno, function_name, code = stack[-2]
code will contain the line of code that was used to call func (in your example, it would be the string func(foobar)). You can parse that to pull out the argument

You can't. It's evaluated before being passed to the function. All you can do is pass it as a string.

#Ivo Wetzel's answer works in the case of function call are made in one line, like
e = 1 + 7
c = 3
foo(e, 100, b=c)
In case that function call is not in one line, like:
e = 1 + 7
c = 3
foo(e,
1000,
b = c)
below code works:
import inspect, ast
def foo(a, f, b):
frame = inspect.currentframe()
frame = inspect.getouterframes(frame)[1]
string = inspect.findsource(frame[0])[0]
nodes = ast.parse(''.join(string))
i_expr = -1
for (i, node) in enumerate(nodes.body):
if hasattr(node, 'value') and isinstance(node.value, ast.Call)
and hasattr(node.value.func, 'id') and node.value.func.id == 'foo' # Here goes name of the function:
i_expr = i
break
i_expr_next = min(i_expr + 1, len(nodes.body)-1)
lineno_start = nodes.body[i_expr].lineno
lineno_end = nodes.body[i_expr_next].lineno if i_expr_next != i_expr else len(string)
str_func_call = ''.join([i.strip() for i in string[lineno_start - 1: lineno_end]])
params = str_func_call[str_func_call.find('(') + 1:-1].split(',')
print(params)
You will get:
[u'e', u'1000', u'b = c']
But still, this might break.

You can use python-varname package
from varname import nameof
s = 'Hey!'
print (nameof(s))
Output:
s
Package below:
https://github.com/pwwang/python-varname

For posterity, here's some code I wrote for this task, in general I think there is a missing module in Python to give everyone nice and robust inspection of the caller environment. Similar to what rlang eval framework provides for R.
import re, inspect, ast
#Convoluted frame stack walk and source scrape to get what the calling statement to a function looked like.
#Specifically return the name of the variable passed as parameter found at position pos in the parameter list.
def _caller_param_name(pos):
#The parameter name to return
param = None
#Get the frame object for this function call
thisframe = inspect.currentframe()
try:
#Get the parent calling frames details
frames = inspect.getouterframes(thisframe)
#Function this function was just called from that we wish to find the calling parameter name for
function = frames[1][3]
#Get all the details of where the calling statement was
frame,filename,line_number,function_name,source,source_index = frames[2]
#Read in the source file in the parent calling frame upto where the call was made
with open(filename) as source_file:
head=[source_file.next() for x in xrange(line_number)]
source_file.close()
#Build all lines of the calling statement, this deals with when a function is called with parameters listed on each line
lines = []
#Compile a regex for matching the start of the function being called
regex = re.compile(r'\.?\s*%s\s*\(' % (function))
#Work backwards from the parent calling frame line number until we see the start of the calling statement (usually the same line!!!)
for line in reversed(head):
lines.append(line.strip())
if re.search(regex, line):
break
#Put the lines we have groked back into sourcefile order rather than reverse order
lines.reverse()
#Join all the lines that were part of the calling statement
call = "".join(lines)
#Grab the parameter list from the calling statement for the function we were called from
match = re.search('\.?\s*%s\s*\((.*)\)' % (function), call)
paramlist = match.group(1)
#If the function was called with no parameters raise an exception
if paramlist == "":
raise LookupError("Function called with no parameters.")
#Use the Python abstract syntax tree parser to create a parsed form of the function parameter list 'Name' nodes are variable names
parameter = ast.parse(paramlist).body[0].value
#If there were multiple parameters get the positional requested
if type(parameter).__name__ == 'Tuple':
#If we asked for a parameter outside of what was passed complain
if pos >= len(parameter.elts):
raise LookupError("The function call did not have a parameter at postion %s" % pos)
parameter = parameter.elts[pos]
#If there was only a single parameter and another was requested raise an exception
elif pos != 0:
raise LookupError("There was only a single calling parameter found. Parameter indices start at 0.")
#If the parameter was the name of a variable we can use it otherwise pass back None
if type(parameter).__name__ == 'Name':
param = parameter.id
finally:
#Remove the frame reference to prevent cyclic references screwing the garbage collector
del thisframe
#Return the parameter name we found
return param

If you want a Key Value Pair relationship, maybe using a Dictionary would be better?
...or if you're trying to create some auto-documentation from your code, perhaps something like Doxygen (http://www.doxygen.nl/) could do the job for you?

I wondered how IceCream solves this problem. So I looked into the source code and came up with the following (slightly simplified) solution. It might not be 100% bullet-proof (e.g. I dropped get_text_with_indentation and I assume exactly one function argument), but it works well for different test cases. It does not need to parse source code itself, so it should be more robust and simpler than previous solutions.
#!/usr/bin/env python3
import inspect
from executing import Source
def func(var):
callFrame = inspect.currentframe().f_back
callNode = Source.executing(callFrame).node
source = Source.for_frame(callFrame)
expression = source.asttokens().get_text(callNode.args[0])
print(expression, '=', var)
i = 1
f = 2.0
dct = {'key': 'value'}
obj = type('', (), {'value': 42})
func(i)
func(f)
func(s)
func(dct['key'])
func(obj.value)
Output:
i = 1
f = 2.0
s = string
dct['key'] = value
obj.value = 42
Update: If you want to move the "magic" into a separate function, you simply have to go one frame further back with an additional f_back.
def get_name_of_argument():
callFrame = inspect.currentframe().f_back.f_back
callNode = Source.executing(callFrame).node
source = Source.for_frame(callFrame)
return source.asttokens().get_text(callNode.args[0])
def func(var):
print(get_name_of_argument(), '=', var)

If you want to get the caller params as in #Matt Oates answer answer without using the source file (ie from Jupyter Notebook), this code (combined from #Aeon answer) will do the trick (at least in some simple cases):
def get_caller_params():
# get the frame object for this function call
thisframe = inspect.currentframe()
# get the parent calling frames details
frames = inspect.getouterframes(thisframe)
# frame 0 is the frame of this function
# frame 1 is the frame of the caller function (the one we want to inspect)
# frame 2 is the frame of the code that calls the caller
caller_function_name = frames[1][3]
code_that_calls_caller = inspect.findsource(frames[2][0])[0]
# parse code to get nodes of abstract syntact tree of the call
nodes = ast.parse(''.join(code_that_calls_caller))
# find the node that calls the function
i_expr = -1
for (i, node) in enumerate(nodes.body):
if _node_is_our_function_call(node, caller_function_name):
i_expr = i
break
# line with the call start
idx_start = nodes.body[i_expr].lineno - 1
# line with the end of the call
if i_expr < len(nodes.body) - 1:
# next expression marks the end of the call
idx_end = nodes.body[i_expr + 1].lineno - 1
else:
# end of the source marks the end of the call
idx_end = len(code_that_calls_caller)
call_lines = code_that_calls_caller[idx_start:idx_end]
str_func_call = ''.join([line.strip() for line in call_lines])
str_call_params = str_func_call[str_func_call.find('(') + 1:-1]
params = [p.strip() for p in str_call_params.split(',')]
return params
def _node_is_our_function_call(node, our_function_name):
node_is_call = hasattr(node, 'value') and isinstance(node.value, ast.Call)
if not node_is_call:
return False
function_name_correct = hasattr(node.value.func, 'id') and node.value.func.id == our_function_name
return function_name_correct
You can then run it as this:
def test(*par_values):
par_names = get_caller_params()
for name, val in zip(par_names, par_values):
print(name, val)
a = 1
b = 2
string = 'text'
test(a, b,
string
)
to get the desired output:
a 1
b 2
string text

Since you can have multiple variables with the same content, instead of passing the variable (content), it might be safer (and will be simpler) to pass it's name in a string and get the variable content from the locals dictionary in the callers stack frame. :
def displayvar(name):
import sys
return name+" = "+repr(sys._getframe(1).f_locals[name])

If it just so happens that the variable is a callable (function), it will have a __name__ property.
E.g. a wrapper to log the execution time of a function:
def time_it(func, *args, **kwargs):
start = perf_counter()
result = func(*args, **kwargs)
duration = perf_counter() - start
print(f'{func.__name__} ran in {duration * 1000}ms')
return result

Why local trace function shows line event for a function call?

I was trying to understand how functions can be traced by following the code mentioned in this article.
import sys
def trace_lines(frame, event, arg):
if event != 'line':
return
co = frame.f_code
func_name = co.co_name
line_no = frame.f_lineno
filename = co.co_filename
print(f"{func_name} line {line_no}")
def trace_calls(frame, event, arg):
if event != 'call':
return
co = frame.f_code
func_name = co.co_name
if func_name == 'write':
# Ignore write() calls from print statements
return
line_no = frame.f_lineno
filename = co.co_filename
print(f"Call to {func_name} on line {line_no} of {filename}")
if func_name in TRACE_INTO:
# Trace into this function
return trace_lines
return
def c(input):
print('input =', input)
print('Leaving c()')
def b(arg):
val = arg * 5
c(val)
print('Leaving b()')
def a():
b(2)
print('Leaving a()')
TRACE_INTO = ['b']
sys.settrace(trace_calls)
a()
Upon running this program, this is the output that we get:
$ python3 test.py
Call to a on line 37 of /Users/xyz/personal/projects/test.py
Call to b on line 32 of /Users/xyz/personal/projects/test.py
b line 33
b line 34
Call to c on line 28 of /Users/xyz/personal/projects/test.py
input = 10
Leaving c()
b line 35
Leaving b()
Leaving a()
Looking at the program, the first two lines of the output are straightforward. The function a() is called which calls function b(). For b(), a local trace function named trace_lines() is used.
Since the first line in the definition of function b() is an expression, it is treated as a line event and therefore, the b line 33 is printed.
But the second line in function b() is a function call to c() and not an expression. So I was expecting it to be a call event. Since the local trace function for function b() is trace_lines(), it should have returned because of the failure of the first condition if event != 'line'. But here, for a function call to c(), the output first shows a line event to trace_lines() and then a call event to trace_calls().
b line 34
Call to c on line 28 of /Users/xyz/personal/projects/test.py
If this is how it works for every function call, Every encounter of a function call should have looked like this-
Step 1: Pass it as a line event
Step 2: Once it is identified as a function, pass a call event to global trace function.
Here is my question-
Why a line event is generated for the function call to c() before the actual call event to global trace function? Also It would be great if somebody can break this output down with explanation.

Prevent 'try' to catch an exception and pass to the next line in python

I have a python function that runs other functions.
def main():
func1(a,b)
func2(*args,*kwargs)
func3()
Now I want to apply exceptions on main function. If there was an exception in any of the functions inside main, the function should not stop but continue executing next line. In other words, I want the below functionality
def main():
try:
func1()
except:
pass
try:
func2()
except:
pass
try:
func3()
except:
pass
So is there any way to loop through each statement inside main function and apply exceptions on each line.
for line in main_function:
try:
line
except:
pass
I just don't want to write exceptions inside the main function.
Note : How to prevent try catching every possible line in python? this question comes close to solving this problem, but I can't figure out how to loop through lines in a function.
If you have any other way to do this other than looping, that would help too.

What you want is on option that exists in some languages where an exception handler can choose to proceed on next exception. This used to lead to poor code and AFAIK has never been implemented in Python. The rationale behind is that you must explicitely say how you want to process an exception and where you want to continue.
In your case, assuming that you have a function called main that only calls other function and is generated automatically, my advice would be to post process it between its generation and its execution. The inspect module can even allow to do it at run time:
def filter_exc(func):
src = inspect.getsource(func)
lines = src.split('\n')
out = lines[0] + "\n"
for line in lines[1:]:
m = re.match('(\s*)(.*)', line)
lead, text = m.groups()
# ignore comments and empty lines
if not (text.startswith('#') or text.strip() == ""):
out += lead + "try:\n"
out += lead + " " + text + "\n"
out += lead + "except:\n" + lead + " pass\n"
return out
You can then use the evil exec (the input in only the source from your function):
exec(filter_exc(main)) # replaces main with the filtered version
main() # will ignore exceptions
After your comment, you want a more robust solution that can cope with multi line statements and comments. In that case, you need to actually parse the source and modify the parsed tree. ast module to the rescue:
class ExceptFilter(ast.NodeTransformer):
def visit_Expr(self, node):
self.generic_visit(node)
if isinstance(node.value, ast.Call): # filter all function calls
# print(node.value.func.id)
# use a dummy try block
n = ast.parse("""try:
f()
except:
pass""").body[0]
n.body[0] = node # make the try call the real function
return n # and use it
return node # keep other nodes unchanged
With that example code:
def func1():
print('foo')
def func2():
raise Exception("Test")
def func3(x):
print("f3", x)
def main():
func1()
# this is a comment
a = 1
if a == 1: # this is a multi line statement
func2()
func3("bar")
we get:
>>> node = ast.parse(inspect.getsource(main))
>>> exec(compile(ExceptFilter().visit(node), "", mode="exec"))
>>> main()
foo
f3 bar
In that case, the unparsed node(*) write as:
def main():
try:
func1()
except:
pass
a = 1
if (a == 1):
try:
func2()
except:
pass
try:
func3('bar')
except:
pass
Alternatively it is also possible to wrap every top level expression:
>>> node = ast.parse(inspect.getsource(main))
>>> for i in range(len(node.body[0].body)): # process top level expressions
n = ast.parse("""try:
f()
except:
pass""").body[0]
n.body[0] = node.body[0].body[i]
node.body[0].body[i] = n
>>> exec(compile(node, "", mode="exec"))
>>> main()
foo
f3 bar
Here the unparsed tree writes:
def main():
try:
func1()
except:
pass
try:
a = 1
except:
pass
try:
if (a == 1):
func2()
except:
pass
try:
func3('bar')
except:
pass
BEWARE: there is an interesting corner case if you use exec(compile(... in a function. By default exec(code) is exec(code, globals(), locals()). At top level, local and global dictionary is the same dictionary, so the top level function is correctly replaced. But if you do the same in a function, you only create a local function with the same name that can only be called from the function (it will go out of scope when the function will return) as locals()['main'](). So you must either alter the global function by passing explicitely the global dictionary:
exec(compile(ExceptFilter().visit(node), "", mode="exec"), globals(), globals())
or return the modified function without altering the original one:
def myfun():
# print(main)
node = ast.parse(inspect.getsource(main))
exec(compile(ExceptFilter().visit(node), "", mode="exec"))
# print(main, locals()['main'], globals()['main'])
return locals()['main']
>>> m2 = myfun()
>>> m2()
foo
f3 bar
(*) Python 3.6 contains an unparser in Tools/parser, but a simpler to use version exists in pypi

You could use a callback, like this:
def main(list_of_funcs):
for func in list_of_funcs:
try:
func()
except Exception as e:
print(e)
if __name__ == "__main__":
main([func1, func2, func3])

Creating functions dynamically in Python -- f(2) cannot resolve (f1)

I am trying to dynamically create 2 functions defined within a string. Code:
def main():
fns = '''
def plus_one(x):
return x + 1
def plus_two(x):
return plus_one(x) + 1
'''
exec(fns)
result = eval('plus_two(11)')
print(result)
if __name__ == '__main__':
main()
Saving this code to a file called dyn_code.py and running it gives me the following error:
python dyn_code.py
Traceback (most recent call last):
File "dyn_code.py", line 19, in <module>
main()
File "dyn_code.py", line 14, in main
result = eval('plus_two(11)')
File "<string>", line 1, in <module>
File "<string>", line 7, in plus_two
NameError: name 'plus_one' is not defined
Problem here is that plus_one cannot be resolved inside plus_two.
plus_one on its own is fine here and can be called with the correct result.
Can anybody please give me an idea on how to inject code like this into the local namespace? Specifically, I want to create 2 functions, with one referring to the other.
I have intentionally used the most open form of both exec and eval, I do know how to restrict them, etc. I have also verified that after the call to exec both functions are present in the local namespace.
What makes this more frustrating is that the code works fine in an interpreter session! That is, after injecting these 2 functions into the interpreter namespace via exec, plus_two runs without any issues.
Ideally, I would like to avoid a function-in-function scenario i.e.
def plus_two(x):
def plus_one(x):
return x + 1
return plus_one(x) + 1
This technique actually works but I want 2 explicitly named and standalone functions.

indentations of your function in fns matters! and you have to pass globals() optional argument for mapping!
def main():
fns = '''def plus_one(x):
return x + 1
def plus_two(x):
return plus_one(x) + 1
'''
exec(fns,globals())
result = eval('plus_two(11)')
print(result)
if __name__ == '__main__':
main()
Output:
13
Hope it helps!

You need to add the globals() dictionary in your call to exec(). You can also omit the eval call for plus_two, like so:
def main():
exec('def plus_one(x):\n return x + 1\n\ndef plus_two(x): return plus_one(x) + 1', globals())
print(plus_two(11))
if __name__ == '__main__':
main()

Get value of last expression in `exec` call

Let's say I have some python code in a string
code = """
a = 42
a
"""
and I exec that string of code:
result = exec(code)
Then result will always be None. Is there any way at all to get the value of the last expression evaluated? In this case, that would be 5, since a was the last expression.
EDIT: Here's another example of the functionality I'm asking about. Let's say we have the python code (stored in the variable code)
a = 100
sqrt(a)
Then how can I execute the code in such a way as to give me the result 10 - that is, sqrt(a)?
EDIT EDIT: A further example: the code I wish to exec is
function_a()
function_b()
function_c()
Is there any way I can define some kind of magic_exec function so that
magic_exec(code)
will provide me with the value of function_c()?

The request is certainly valid because I need such a function as well during the creation of a Python-based environment. I solved the problem with the following code that utilizes the Python ast mechanism:
def my_exec(script, globals=None, locals=None):
'''Execute a script and return the value of the last expression'''
stmts = list(ast.iter_child_nodes(ast.parse(script)))
if not stmts:
return None
if isinstance(stmts[-1], ast.Expr):
# the last one is an expression and we will try to return the results
# so we first execute the previous statements
if len(stmts) > 1:
exec(compile(ast.Module(body=stmts[:-1]), filename="<ast>", mode="exec"), globals, locals)
# then we eval the last one
return eval(compile(ast.Expression(body=stmts[-1].value), filename="<ast>", mode="eval"), globals, locals)
else:
# otherwise we just execute the entire code
return exec(script, globals, locals)
The code should be pretty self-explanatory, basically it
separate the script into multiple statements
if the last one is an expression, execute the first part as statements, and the last part as expression.
Otherwise execute the entire script as statements.

This doesn't get you the last evaluated value, but gets the whole list of local variables.
>>> loc = {}
>>> exec(code, {}, loc)
>>> loc
{'a': 42}

exec('a = 4')
print a % prints 4
>>> code = """
... a = 42
... b = 53"""
>>> exec(code)
>>> a
42
>>> b
53
Or if you're saying you don't know the last thing is b for instance, then you can have this:
code = """
a = 4
b = 12
abc_d=13
"""
t = re.findall(r'''.*?([A-Za-z0-9_]+)\s*?=.*?$''', code)
assert(len(t)==1)
print t[0] % prints 13

To be honest I can't say I'm very happy with this. It feels very hacky and I haven't tested it all that heavily. On the other hand I'm quite pleased with it. Was quite fun to do. Anyway, hope this helps you or at least comes close to what you want. locals() gives a dict so the output list order does not match the input order for the items that failed the first eval. If you don't want ';' as delimiters then you can change it to '\n'.
import math
def magic_exec(_command):
_command = _command.split(';')
_result = None
_before = list(locals()) # Get list of current local variables
for _code in _command:
_code = _code.strip() # .strip() prevent IndentationError
try:
if eval(_code) != None: # For functions with no return
_result = eval(_code)
except (NameError, SyntaxError):
try:
_before = list(locals())
exec(_code)
except NameError as e: # For undefined variables in _command
print("An Error Occurred with line ' {0} ' as was skipped: {1}".format(_code, e))
del _code # delete temp var _code
# Get new list of locals that didn't exist at the start
_after = [val for val in list(locals()) if val not in _before]
if _after:
return eval(_after[0])
else:
return _result
#Dummy class and functions
class Class1(object):
def __init__(self, x):
self._x = x
def get_val(self):
return self._x
def __repr__(self):
return type(self).__name__
def func1(x):
return x + x
def func2(x):
print(x*x)
if __name__ == '__main__':
code = \
"""
a = 42; a; v; y = 2; b = func1(5); s = 'Hello'; func2(10); c = 25; l = []; l.append('Value');
t = math.sqrt(c); pass; 20*10; print('TEST'); math.sqrt(c); d = Class1('World'); d.get_val();
def func3(x): return x ** 2; s = func3(15)
"""
values = magic_exec(code)
print(values)

I would like to add to user2283347's excellent answer that it works only up to Python 3.7. In Python 3.8 the signature of ast.Module.__init__ has changed. It now requires a second argument which in our case can be an empty list.
Details: ast.Module(body=stmts[:-1]) in
if len(stmts) > 1:
exec(compile(ast.Module(body=stmts[:-1]), filename="<ast>", mode="exec"), globals, locals)
has to be changed to
ast.Module(stmts[:-1], []) if you use Python 3.8 or above (note the second argument []). Otherwise the following TypeError will be raised:
TypeError: required field "type_ignores" missing from Module
Unfortunately this change is not very well documented. I found the solution after extensive Googling here: "IPython broken on 3.8-dev" .