Avoid exec in instance variable definition

Question

I have a class in which I find myself define similar groups of variables many times, to the point that I believe it would be clearer if I were able to do so in a loop, as follows:

import tkinter as tk
from tkinter import ttk

class MyClass:
    def __init__(self):
        self.initializeVariables()

    def initializeVariables(self):
        varnames = ['a', 'b', 'c', 'd']
        for var in varnames:
            exec("self.bg{0} = ttk.Frame(self.Frame)".format(var))
            exec("self.tb{0} = ttk.Entry(bg{0}, width=4)".format(var))
            exec("self.tb{0}.grid(row=i)".format(var))

I have more elements than this, this example is just meant to be illustrative. I would like to know:

Is there a way to accomplish this without using the exec function? I tried with globals(), but was not successful
Is doing this inadvisable? In my opinion, this makes the code easier to read, but I am not aware of typical conventions in this arena.

Just setattr(self, 'tb' + var, ttk.Entry(self.Frame, width=4) — vaultah
– vaultah, Commented Sep 18, 2015 at 14:34

Bryan Oakley · Accepted Answer · 2015-09-18 14:55:30Z

3

There's no need to have individual object attributes for each widget. Creating attributes in a loop adds complexity without providing any real value in this case.

I suggest using a list or dictionary to store the references:

def initializeVariables(self):
    varnames = ['a', 'b', 'c', 'd']
    self.frames = {}
    self.entries = {}
    for var in varnames:
        self.frames[var] = ttk.Frame(...)
        self.entries[var] = ttk.Entry(...)

You can then later access them by their name. for example:

self.entries["c"].get()

Unless you need to access the frames in other parts of the code, you can use a local variable for the frame:

for var in varnames:
    frame = tk.Frame(...)
    self.entries[var] = ttk.Entry(frame, ...)

edited Sep 18, 2015 at 14:55

answered Sep 18, 2015 at 14:48

Bryan Oakley

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1 Comment

wesanyer Over a year ago

Accepting this answer based on the comments from Martijn Pieter's answer.

Martijn Pieters · Accepted Answer · 2015-09-18 14:55:25Z

2

You have several options far better than exec:

Use the setattr() function to set arbitrary attributes, given a string name for the attribute:

varnames = ['a', 'b', 'c', 'd']
for var in varnames:
    frame = ttk.Frame(self.Frame)
    entry = ttk.Entry(frame, width=4)
    entry.grid(row=i)
    setattr(self, 'bg{0}'.format(var), frame)
    setattr(self, 'tb{0}'.format(var), entry)

Use vars(self) to access the instance namespace as a dictionary:

namespace = vars(self)
varnames = ['a', 'b', 'c', 'd']
for var in varnames:
    frame = ttk.Frame(self.Frame)
    entry = ttk.Entry(frame, width=4)
    entry.grid(row=i)
    namespace['bg{0}'.format(var)] = frame
    namespace['tb{0}'.format(var)] = entry

Access the self.__dict__ namespace directly:

varnames = ['a', 'b', 'c', 'd']
for var in varnames:
    frame = ttk.Frame(self.Frame)
    entry = ttk.Entry(frame, width=4)
    entry.grid(row=i)
    self.__dict__['bg{0}'.format(var)] = frame
    self.__dict__['tb{0}'.format(var)] = entry

vars(self) is basically a forward-compatible and API-friendly way of spelling self.__dict__. I prefer using setattr() as this is compatible with classes using __slots__ or hooking into __setattr__.

Setting attributes in a loop is fine (great even, as you are applying the DRY principle), using exec is not the best way to go about this however.

Do consider using a dictionary or list for repeated or grouped attributes. A dictionary or list makes it much easier to later access the whole group of elements in go:

frames = self.frames = {}
framed_entries = self.framed_entries = {}
varnames = ['a', 'b', 'c', 'd']
for var in varnames:
    frame = ttk.Frame(self.Frame)
    entry = ttk.Entry(frame, width=4)
    entry.grid(row=i)
    frames['bg{0}'.format(var)] = frame
    framed_entries['tb{0}'.format(var)] = entry

Now you can access those same frames and entries with a simple loop over a dictionary, or directly address each one with a suitable key.

edited Sep 18, 2015 at 14:55

answered Sep 18, 2015 at 14:34

Martijn Pieters

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5 Comments

wesanyer Over a year ago

This is great, thank you! However, can you please address my second question too?

Martijn Pieters Over a year ago

@wesanyer: there, added a sentence.

wesanyer Over a year ago

Perfect. I will accept this as an answer as soon as SO allows me too. Thanks again.

Bryan Oakley Over a year ago

While I think this is a good, literal answe for the question that was asked, I think creating object attributes in this way will lead to hard-to-understand code.

Martijn Pieters Over a year ago

@BryanOakley: true; using a dictionary or list lets you group the whole lot under a common 'namespace' separate from the instance.

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