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Problems implementing a deque class with output Deque<elements>

I'm attending a python course in university, where we should implement a deque class as an assignment. Now I've got some problems with the output an instance should give given in the docstring.
class Deque:
"""
>>> d = Deque()
>>> d
Deque<>
>>> d = d.append(1); d
Deque<1>
>>> d.append(2).prepend(0) # allow for chaining of appending & prepending
Deque<0, 1, 2>
I have no idea how to achieve that an instance outputs this notation with the angle brackets.
Anybody got an idea?
Thanks in advance

You work can work that out in the __repr__ of the class.
As a simple example for an empty Deque instance:
>>> class Deque(object):
... def __repr__(self):
... return 'Deque<>'
...
>>> d = Deque()
>>> d
Deque<>
For a non-empty instance, you would simply format the returned string to include the contents of the instance.

You need to define the __repr__() method; you can return a string formatted however you want.

How do I get the return value when using Python exec on the code object of a function?

For testing purposes I want to directly execute a function defined inside of another function.
I can get to the code object of the child function, through the code (func_code) of the parent function, but when I exec it, i get no return value.
Is there a way to get the return value from the exec'ed code?

Yes, you need to have the assignment within the exec statement:
>>> def foo():
... return 5
...
>>> exec("a = foo()")
>>> a
5
This probably isn't relevant for your case since its being used in controlled testing, but be careful with using exec with user defined input.

A few years later, but the following snippet helped me:
the_code = '''
a = 1
b = 2
return_me = a + b
'''
loc = {}
exec(the_code, globals(), loc)
return_workaround = loc['return_me']
print(return_workaround) # 3
exec() doesn't return anything itself, but you can pass a dict which has all the local variables stored in it after execution. By accessing it you have a something like a return.
I hope it helps someone.

While this is the ugliest beast ever seen by mankind, this is how you can do it by using a global variable inside your exec call:
def my_exec(code):
exec('global i; i = %s' % code)
global i
return i
This is misusing global variables to get your data across the border.
>>> my_exec('1 + 2')
3
Needless to say that you should never allow any user inputs for the input of this function in there, as it poses an extreme security risk.

use eval() instead of exec(), it returns result

Something like this can work:
def outer():
def inner(i):
return i + 10
for f in outer.func_code.co_consts:
if getattr(f, 'co_name', None) == 'inner':
inner = type(outer)(f, globals())
# can also use `types` module for readability:
# inner = types.FunctionType(f, globals())
print inner(42) # 52
The idea is to extract the code object from the inner function and create a new function based on it.
Additional work is required when an inner function can contain free variables. You'll have to extract them as well and pass to the function constructor in the last argument (closure).

Here's a way to return a value from exec'd code:
def exec_and_return(expression):
exec(f"""locals()['temp'] = {expression}""")
return locals()['temp']
I'd advise you to give an example of the problem you're trying to solve. Because I would only ever use this as a last resort.

This doesn't get the return value per say, but you can provide an empty dictionary when calling exec to retrieve any variables defined in the code.
# Python 3
ex_locals = {}
exec("a = 'Hello world!'", None, ex_locals)
print(ex_locals['a'])
# Output: Hello world!
From the Python 3 documentation on exec:
The default locals act as described for function locals() below: modifications to the default locals dictionary should not be attempted. Pass an explicit locals dictionary if you need to see effects of the code on locals after function exec() returns.
For more information, see How does exec work with locals?

Here's a solution with a simple code:
# -*- coding: utf-8 -*-
import math
x = [0]
exec("x[0] = 3*2")
print(x[0]) # 6

Since Python 3.7, dictionary are ordered. So you no longer need to agree on a name, you can just say "last item that got created":
>>> d = {}
>>> exec("def addone(i): return i + 1", d, d)
>>> list(d)
['__builtins__', 'addone']
>>> thefunction = d[list(d)[-1]]
>>> thefunction
<function addone at 0x7fd03123fe50>

if we need a function that is in a file in another directory, eg
we need the function1 in file my_py_file.py
located in /home/.../another_directory
we can use the following code:
def cl_import_function(a_func,py_file,in_Dir):
... import sys
... sys.path.insert(0, in_Dir)
... ax='from %s import %s'%(py_file,a_func)
... loc={}
... exec(ax, globals(), loc)
... getFx = loc[afunc]
... return getFx
test = cl_import_function('function1',r'my_py_file',r'/home/.../another_directory/')
test()
(a simple way for newbies...)

program = 'a = 5\nb=10\nprint("Sum =", a + b)'
program = exec(program)
print(program)

python string to code for function arg?

I have a function names "myfunction". I have a string "a", I need to pass it to "myfunction" so it will give the same result as if my string was a python object name, myfunction(a)
So I have
def myfunction(var):
print var
a = 1
mystring = "a"
I need to pass "mystring" to "myfuntion" so it will behave as variable "a" was passed to it.
I thought of something like this, but it won't work:
myfunction(exec(mystring))
PS. Yes, I know of the consequences of exec(), please there is no need to explain that.

It would be eval(), not exec:
myfunction(eval(mystring))
Alternatively:
myfunction(locals()[mystring])
Most probably you have a fundamental design problem if you think you need something like this.

No needs in exec or eval:
>>> def myfunc(var):
... print globals()[var] * 2
...
>>> a = 12
>>> myfunc('a')
24

How to retrieve a variable's name in python at runtime?

Is there a way to know, during run-time, a variable's name (from the code)?
Or do variable's names forgotten during compilation (byte-code or not)?
e.g.:
>>> vari = 15
>>> print vari.~~name~~()
'vari'
Note: I'm talking about plain data-type variables (int, str, list etc.)

Variable names don't get forgotten, you can access variables (and look which variables you have) by introspection, e.g.
>>> i = 1
>>> locals()["i"]
1
However, because there are no pointers in Python, there's no way to reference a variable without actually writing its name. So if you wanted to print a variable name and its value, you could go via locals() or a similar function. ([i] becomes [1] and there's no way to retrieve the information that the 1 actually came from i.)

Variable names persist in the compiled code (that's how e.g. the dir built-in can work), but the mapping that's there goes from name to value, not vice versa. So if there are several variables all worth, for example, 23, there's no way to tell them from each other base only on the value 23 .

Here is a function I use to print the value of variables, it works for local as well as globals:
import sys
def print_var(var_name):
calling_frame = sys._getframe().f_back
var_val = calling_frame.f_locals.get(var_name, calling_frame.f_globals.get(var_name, None))
print (var_name+':', str(var_val))
So the following code:
global_var = 123
def some_func():
local_var = 456
print_var("global_var")
print_var("local_var")
print_var("some_func")
some_func()
produces:
global_var: 123
local_var: 456
some_func: <function some_func at 0x10065b488>

here a basic (maybe weird) function that shows the name of its argument...
the idea is to analyze code and search for the calls to the function (added in the init method it could help to find the instance name, although with a more complex code analysis)
def display(var):
import inspect, re
callingframe = inspect.currentframe().f_back
cntext = "".join(inspect.getframeinfo(callingframe, 5)[3]) #gets 5 lines
m = re.search("display\s+\(\s+(\w+)\s+\)", cntext, re.MULTILINE)
print m.group(1), type(var), var
please note:
getting multiple lines from the calling code helps in case the call was split as in the below example:
display(
my_var
)
but will produce unexpected result on this:
display(first_var)
display(second_var)
If you don't have control on the format of your project you can still improve the code to detect and manage different situations...
Overall I guess a static code analysis could produce a more reliable result, but I'm too lazy to check it now

This will work for simple data types (str, int, float, list etc.)
def my_print(var_str) :
print var_str+':', globals()[var_str]

You can do it, it's just not pretty.
import inspect, sys
def addVarToDict(d, variable):
lineNumber = inspect.currentframe().f_back.f_lineno
with open(sys.argv[0]) as f:
lines = f.read().split("\n")
line = lines[lineNumber-1]
varName = line.split("addVarToDict")[1].split("(")[1].split(",")[1].split(")")[0].strip()
d[varName] = variable
d = {}
a=1
print d # {}
addVarToDict(d,a)
print d # {'a': 1}

I tried the following link from the post above with no success:
Googling returned this one.
http://pythonic.pocoo.org/2009/5/30/finding-objects-names

Just yesterday I saw a blog post with working code that does just this. Here's the link:
http://pyside.blogspot.com/2009/05/finding-objects-names.html

Nice easy solution using f-string formatting, which is native to Python 3.6 and later:
vari = 15
vari_name = f"{vari=}".split("=")[0]
print(vari_name)
Produces:
vari

How can I get the name of an object?

Suppose I have code like:
x = 0
y = 1
z = 2
my_list = [x, y, z]
for item in my_list:
print("handling object ", name(item)) # <--- what would go instead of `name`?
How can I get the name of each object in Python? That is to say: what could I write instead of name in this code, so that the loop will show handling object x and then handling object y and handling object z?
In my actual code, I have a dict of functions that I will call later after looking them up with user input:
def fun1():
pass
def fun2():
pass
def fun3():
pass
fun_dict = {'fun1': fun1,
'fun2': fun2,
'fun3': fun3}
# suppose that we get the name 'fun3' from the user
fun_dict['fun3']()
How can I create fun_dict automatically, without writing the names of the functions twice? I would like to be able to write something like
fun_list = [fun1, fun2, fun3] # and I'll add more as the need arises
fun_dict = {}
for t in fun_list:
fun_dict[name(t)] = t
to avoid duplicating the names.

Objects do not necessarily have names in Python, so you can't get the name.
When you create a variable, like the x, y, z above then those names just act as "pointers" or "references" to the objects. The object itself does not know what name(s) you are using for it, and you can not easily (if at all) get the names of all references to that object.
However, it's not unusual for objects to have a __name__ attribute. Functions do have a __name__ (unless they are lambdas), so we can build fun_dict by doing e.g.
fun_dict = {t.__name__: t for t in fun_list)

That's not really possible, as there could be multiple variables that have the same value, or a value might have no variable, or a value might have the same value as a variable only by chance.
If you really want to do that, you can use
def variable_for_value(value):
for n,v in globals().items():
if v == value:
return n
return None
However, it would be better if you would iterate over names in the first place:
my_list = ["x", "y", "z"] # x, y, z have been previously defined
for name in my_list:
print "handling variable ", name
bla = globals()[name]
# do something to bla

This one-liner works, for all types of objects, as long as they are in globals() dict, which they should be:
def name_of_global_obj(xx):
return [objname for objname, oid in globals().items()
if id(oid)==id(xx)][0]
or, equivalently:
def name_of_global_obj(xx):
for objname, oid in globals().items():
if oid is xx:
return objname

As others have mentioned, this is a really tricky question. Solutions to this are not "one size fits all", not even remotely. The difficulty (or ease) is really going to depend on your situation.
I have come to this problem on several occasions, but most recently while creating a debugging function. I wanted the function to take some unknown objects as arguments and print their declared names and contents. Getting the contents is easy of course, but the declared name is another story.
What follows is some of what I have come up with.
Return function name
Determining the name of a function is really easy as it has the __name__ attribute containing the function's declared name.
name_of_function = lambda x : x.__name__
def name_of_function(arg):
try:
return arg.__name__
except AttributeError:
pass`
Just as an example, if you create the function def test_function(): pass, then copy_function = test_function, then name_of_function(copy_function), it will return test_function.
Return first matching object name
Check whether the object has a __name__ attribute and return it if so (declared functions only). Note that you may remove this test as the name will still be in globals().
Compare the value of arg with the values of items in globals() and return the name of the first match. Note that I am filtering out names starting with '_'.
The result will consist of the name of the first matching object otherwise None.
def name_of_object(arg):
# check __name__ attribute (functions)
try:
return arg.__name__
except AttributeError:
pass
for name, value in globals().items():
if value is arg and not name.startswith('_'):
return name
Return all matching object names
Compare the value of arg with the values of items in globals() and store names in a list. Note that I am filtering out names starting with '_'.
The result will consist of a list (for multiple matches), a string (for a single match), otherwise None. Of course you should adjust this behavior as needed.
def names_of_object(arg):
results = [n for n, v in globals().items() if v is arg and not n.startswith('_')]
return results[0] if len(results) is 1 else results if results else None

If you are looking to get the names of functions or lambdas or other function-like objects that are defined in the interpreter, you can use dill.source.getname from dill. It pretty much looks for the __name__ method, but in certain cases it knows other magic for how to find the name... or a name for the object. I don't want to get into an argument about finding the one true name for a python object, whatever that means.
>>> from dill.source import getname
>>>
>>> def add(x,y):
... return x+y
...
>>> squared = lambda x:x**2
>>>
>>> print getname(add)
'add'
>>> print getname(squared)
'squared'
>>>
>>> class Foo(object):
... def bar(self, x):
... return x*x+x
...
>>> f = Foo()
>>>
>>> print getname(f.bar)
'bar'
>>>
>>> woohoo = squared
>>> plus = add
>>> getname(woohoo)
'squared'
>>> getname(plus)
'add'

Use a reverse dict.
fun_dict = {'fun1': fun1,
'fun2': fun2,
'fun3': fun3}
r_dict = dict(zip(fun_dict.values(), fun_dict.keys()))
The reverse dict will map each function reference to the exact name you gave it in fun_dict, which may or may not be the name you used when you defined the function. And, this technique generalizes to other objects, including integers.
For extra fun and insanity, you can store the forward and reverse values in the same dict. I wouldn't do that if you were mapping strings to strings, but if you are doing something like function references and strings, it's not too crazy.

Note that while, as noted, objects in general do not and cannot know what variables are bound to them, functions defined with def do have names in the __name__ attribute (the name used in def). Also if the functions are defined in the same module (as in your example) then globals() will contain a superset of the dictionary you want.
def fun1:
pass
def fun2:
pass
def fun3:
pass
fun_dict = {}
for f in [fun1, fun2, fun3]:
fun_dict[f.__name__] = f

Here's another way to think about it. Suppose there were a name() function that returned the name of its argument. Given the following code:
def f(a):
return a
b = "x"
c = b
d = f(c)
e = [f(b), f(c), f(d)]
What should name(e[2]) return, and why?

And the reason I want to have the name of the function is because I want to create fun_dict without writing the names of the functions twice, since that seems like a good way to create bugs.
For this purpose you have a wonderful getattr function, that allows you to get an object by known name. So you could do for example:
funcs.py:
def func1(): pass
def func2(): pass
main.py:
import funcs
option = command_line_option()
getattr(funcs, option)()

I know This is late answer.
To get func name , you can use func.__name__
To get the name of any python object that has no name or __name__ method. You can iterate over its module members.
Ex:.
# package.module1.py
obj = MyClass()
# package.module2.py
import importlib
def get_obj_name(obj):
mod = Obj.__module__ # This is necessary to
module = module = importlib.import_module(mod)
for name, o in module.__dict__.items():
if o == obj:
return name
Performance note: don't use it in large modules.

Variable names can be found in the globals() and locals() dicts. But they won't give you what you're looking for above. "bla" will contain the value of each item of my_list, not the variable.

Generally when you are wanting to do something like this, you create a class to hold all of these functions and name them with some clear prefix cmd_ or the like. You then take the string from the command, and try to get that attribute from the class with the cmd_ prefixed to it. Now you only need to add a new function/method to the class, and it's available to your callers. And you can use the doc strings for automatically creating the help text.
As described in other answers, you may be able to do the same approach with globals() and regular functions in your module to more closely match what you asked for.
Something like this:
class Tasks:
def cmd_doit(self):
# do it here
func_name = parse_commandline()
try:
func = getattr('cmd_' + func_name, Tasks())
except AttributeError:
# bad command: exit or whatever
func()

I ran into this page while wondering the same question.
As others have noted, it's simple enough to just grab the __name__ attribute from a function in order to determine the name of the function. It's marginally trickier with objects that don't have a sane way to determine __name__, i.e. base/primitive objects like basestring instances, ints, longs, etc.
Long story short, you could probably use the inspect module to make an educated guess about which one it is, but you would have to probably know what frame you're working in/traverse down the stack to find the right one. But I'd hate to imagine how much fun this would be trying to deal with eval/exec'ed code.
% python2 whats_my_name_again.py
needle => ''b''
['a', 'b']
[]
needle => '<function foo at 0x289d08ec>'
['c']
['foo']
needle => '<function bar at 0x289d0bfc>'
['f', 'bar']
[]
needle => '<__main__.a_class instance at 0x289d3aac>'
['e', 'd']
[]
needle => '<function bar at 0x289d0bfc>'
['f', 'bar']
[]
%
whats_my_name_again.py:
#!/usr/bin/env python
import inspect
class a_class:
def __init__(self):
pass
def foo():
def bar():
pass
a = 'b'
b = 'b'
c = foo
d = a_class()
e = d
f = bar
#print('globals', inspect.stack()[0][0].f_globals)
#print('locals', inspect.stack()[0][0].f_locals)
assert(inspect.stack()[0][0].f_globals == globals())
assert(inspect.stack()[0][0].f_locals == locals())
in_a_haystack = lambda: value == needle and key != 'needle'
for needle in (a, foo, bar, d, f, ):
print("needle => '%r'" % (needle, ))
print([key for key, value in locals().iteritems() if in_a_haystack()])
print([key for key, value in globals().iteritems() if in_a_haystack()])
foo()

You define a class and add the Unicode private function insert the class like
class example:
def __init__(self, name):
self.name = name
def __unicode__(self):
return self.name
Of course you have to add extra variable self.name which is the name of the object.

Here is my answer, I am also using globals().items()
def get_name_of_obj(obj, except_word = ""):
for name, item in globals().items():
if item == obj and name != except_word:
return name
I added except_word because I want to filter off some word used in for loop.
If you didn't add it, the keyword in for loop may confuse this function, sometimes the keyword like "each_item" in the following case may show in the function's result, depends on what you have done to your loop.
eg.
for each_item in [objA, objB, objC]:
get_name_of_obj(obj, "each_item")
eg.
>>> objA = [1, 2, 3]
>>> objB = ('a', {'b':'thi is B'}, 'c')
>>> for each_item in [objA, objB]:
... get_name_of_obj(each_item)
...
'objA'
'objB'
>>>
>>>
>>> for each_item in [objA, objB]:
... get_name_of_obj(each_item)
...
'objA'
'objB'
>>>
>>>
>>> objC = [{'a1':'a2'}]
>>>
>>> for item in [objA, objB, objC]:
... get_name_of_obj(item)
...
'objA'
'item' <<<<<<<<<< --------- this is no good
'item'
>>> for item in [objA, objB]:
... get_name_of_obj(item)
...
'objA'
'item' <<<<<<<<--------this is no good
>>>
>>> for item in [objA, objB, objC]:
... get_name_of_obj(item, "item")
...
'objA'
'objB' <<<<<<<<<<--------- now it's ok
'objC'
>>>
Hope this can help.

Based on what it looks like you're trying to do you could use this approach.
In your case, your functions would all live in the module foo. Then you could:
import foo
func_name = parse_commandline()
method_to_call = getattr(foo, func_name)
result = method_to_call()
Or more succinctly:
import foo
result = getattr(foo, parse_commandline())()

Python has names which are mapped to objects in a hashmap called a namespace. At any instant in time, a name always refers to exactly one object, but a single object can be referred to by any arbitrary number of names. Given a name, it is very efficient for the hashmap to look up the single object which that name refers to. However given an object, which as mentioned can be referred to by multiple names, there is no efficient way to look up the names which refer to it. What you have to do is iterate through all the names in the namespace and check each one individually and see if it maps to your given object. This can easily be done with a list comprehension:
[k for k,v in locals().items() if v is myobj]
This will evaluate to a list of strings containing the names of all local "variables" which are currently mapped to the object myobj.
>>> a = 1
>>> this_is_also_a = a
>>> this_is_a = a
>>> b = "ligma"
>>> c = [2,3, 534]
>>> [k for k,v in locals().items() if v is a]
['a', 'this_is_also_a', 'this_is_a']
Of course locals() can be substituted with any dict that you want to search for names that point to a given object. Obviously this search can be slow for very large namespaces because they must be traversed in their entirety.

Hi there is one way to get the variable name that stores an instance of a class
is to use
locals()
function, it returns a dictionary that contains the variable name as a string and its value