Why do Python's frameworks return dictionaries from controllers?

StackOverflow https://stackoverflow.com/questions/634024

  •  10-07-2019
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Question

Why (for example web2py) do you return data from a controller in a dictionary instead of variables (see Rails)?

For example:

return dict(sape=4139, guido=4127, jack=4098)

instead of (that's the way Rails does it)

@var1 = "jello" 
@var2 = "hihi"

Is there any advantage using dictionaries over plain variables (speed-wise/code-wise)?

Update: The above way is actually a correct way for creating a dictionary (at least in Python 2.6.1). The other way (that many people say it's the correct one)

return {"var1": "jello", "var2": "hihi"}

is not used a lot by python frameworks.

From Python's documentation: "When the keys are simple strings, it is sometimes easier to specify pairs using keyword arguments:"

dict(sape=4139, guido=4127, jack=4098)
Was it helpful?

Solution

The main advantage is that this is the only way in python to return a) more than a single value and b) give that value a name. Other options would be to use a class (extra code), return a tuple (no names, so you'd have to use indexes to access the values) or allow to return only a single value which would probably mean that everyone would return a dictionary since that's the most simple solution.

It also allows to wrap several methods and join/merge their results. Lastly, it allows to return different sets of value/name pairs for each call easily, for example, omit optional values or return additional hints.

OTHER TIPS

You can use local variables if you'd like:

def hello():
    var1 = "whatever you like"
    var2 = "another value"
    return locals() # or vars()

hello.html:

<html><body>
 <p>var1 {{=var1}}</p>
 <p>var2 {{=var2}}</p>
</body></html>

from PHP to web2py:

In web2py an HTTP request for "/app/c/f" is mapped into a call to the function f() in file (controller) c.py in the application "app". The file c.py is written in Python. The output of the function f() can be a string (in this case it is returned), or a set of variables (implemented as a python dictionary). In the latter case the variables are rendered into HTML by a file c/f.html, called a view.

The nice thing is that a template engine like Jinja2 treats an object and a dict similarly, so if:

d = {'color': 'red'}
o = Color(red)

then these all work in the template syntax:

d.color d['color'] o.color o['color']
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