From Ruby to Python - Is there an equivalent of "try"?

user_5032083 0
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In Ruby (on Rails, actually), I have the try function:

[].try(:[], 1) # => nil
[10].try(:[], 1) # => nil
[10, 20].try(:[], 1) # => 20
[10, 20, 30].try(:[], 1) # => 20

{}.try(:foo) # => nil
{ foo: 'bar' }.try(:foo) # => 'bar'

The function basically is a shortcut to avoid ifs to check if the position is present.
Is there something similar in Python?

list dictionary python
7 Answers

Dictionaries: dict.get

You can use dict.get:

d = {'foo' : 'bar'}

print(d.get('foo'))
'bar'

print(d.get('xxxxx'))
None

You can also pass a default parameter to get:

print(d.get('xxxxx', 'Invalid Key!'))
Invalid Key!

The default value is printed out when the key does not exist in the dictionary.

Lists: custom try-except block

Unfortunately, lists do not have a dict.get equivalent in their API, so you'll need to implement one yourself. Thankfully, you can extend the list class and override __getitem__ to do this cleanly.

class MyList(list):
    def __getitem__(self, idx, default='oops'):
        try:
            return super().__getitem__(idx)
        except IndexError:
            return default

l = MyList([10, 20])

l[1]
# 20

l[3]
# 'oops'
user_49090870

It is not possible to have a method like try in Python, since try crucially relies on monkey patching Object and NilClass, and their Python equivalents (object and NoneType) cannot be monkey patched:

def try_object(self, f, *args, **kwargs):
    return getattr(self, f)(*args, **kwargs)

def try_none(self, f, *args, **kwargs):
    return None

object.try   = try_object
object.try   = try_object
#            ^
# SyntaxError: invalid syntax

NoneType.try = try_none
#            ^
# SyntaxError: invalid syntax

However, you can write a global try function that behaves similar:

def tryy(obj, f, *args, **kwargs):
    if obj is None:
        return None
    else:
        return getattr(obj, f)(*args, **kwargs)

tryy([10, 20], '__getitem__', 1)
# >>> 20

tryy(None, '__getitem__', 1)
# >>> None
user_29880

python list object has no safe .get like dictionary has.

you will need to catch that with your own try and except: 

try:
    return your_list[index]
except IndexError:
    return default # could be -1 for example
user_60065840

With dict's you can use d.get('foo') which returns nothing if the key doesn't exist. You also can specify a defualt value, d.get('foo', 'bar') in which case bar is returned if the key foo isn't in the dict.

With lists, there isn't such obvious way. You can define a simple function,

def check_item(list_obj, item):
    try:
        return list_obj[item]
    except IndexError:
        pass

Which will return the value if it exists, and nothing if not. Then simply call it value = check_item(list_item, item_to_look)

More advanced option would be to subclass the list class, and override the __getitem__ method a function with similar logic to the above. (It requires some additional modifications to handle slice objects, etc.)

user_8405820

In python, you must live by this mantra "It's easier to ask forgiveness than get permission".

For a list however, you can write a check like this to make the list into a dict with having its indexes as keys.

>>>en = enumerate([10, 20])
>>>dct = dict(en)
>>>dct.get(2)
None
>>>dct.get(1)
20
user_51898110

combining solutions

def safe_get(data, key):
    if isinstance(data, (list, set, tuple)):
        return dict(enumerate(data)).get(key)
    elif isinstance(data, dict):
        return (data).get(key)
    else:
        return None

print safe_get([], 1)
None
print safe_get([10], 1)
None
print safe_get([10, 20], 1)
20
print safe_get([10, 20, 30], 1)
20
print safe_get({}, 'foo')
None
print safe_get({ 'foo': 'bar' }, 'foo')
bar
user_80601200

you can use getattr which will do what try would do for properties/attributes but it won't work for methods which need to be called.

eg

getattr(getattr(foo, 'prop1', None), 'prop2', None)

this is the same as in ruby

foo.try(:prop1).try(:prop2)
user_5662450
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