Python: Ifelse Conditionals
The regular ifelse statements syntax:
if condition1:
to_do_if_condition1_istrue
elif condition2:
to_do_if_condition2_istrue
else:
to_do_if_no_conditions_istrue
Some alternatives to the regular ifelse statements.
Inline ifelse
Everything is on one line.
expression_if_true if condition else expression_if_false
It looks at the condition.

If it is TRUE, then it does what is on the left.

If it is FALSE, then it does what is on the right.
>>> a = 2
>>> b = 3
>>> 'a is larger' if a > b else 'a is smaller'
'a is smaller'
Another example:
def add(a,b):
return a + b
def subtract(a, b):
return a  b
add(a, b) if mode=='add' else subtract(a, b)
mode = 'add'
a = 4
b = 6
>>>10
mode = ''
>>>2
Inline ifelse EXPRESSION must always contain else
clause.
 But if you want the variable to maintain its original value if the condition is false, then the expression_if_false can be the original variable. For example:
a = 1 if b else a
Inline also works for list comprehensions:
>>> [f if f.isdigit() else 'not digit' for f in ["1", "h", "l", "8", "9"]]
['1', 'not digit', 'not digit', '8', '9']
which is the same as:
flist =[]
for f in ["1", "h", "l", "8", "9"]:
if f.isdigit():
flist.append(f)
else:
flist.append('not digit')
return flist
Nested ifs, as inline
>>>student = True
>>>status = "S"
>>>'Freshie' if status == "F" else "not Freshie" if student == True else 'not a student'
>>>'not Freshie'
>>>student = False
>>>status = ''
>>>'Freshie' if status == "F" else "not Freshie" if student == True else 'not a student'
>>>'not a student'
How this one is evaluated:
First, evaluates the RIGHTmost if statement.

If it is FALSE, it returns the expression_if_false.

If it is TRUE, it looks to the right. In this case it encounters another ifelse statement.
Then ,evaluates the next ifelse.

If it is FALSE, it returns the expression_if_false.

If it is TRUE, it looks to the right. In this case it encounters the expression_if_true. And stops there.
Another way to accomplish nested ifs is with a dictionary
>>> def add(a,b):
... return a + b
...
>>> def multiply(a, b):
... return a * b
...
>>> def subtract(a, b):
... return a  b
...
>>>funcs_dict = {'1': add, '2': multiply, '3':subtract}
>>>s='1'
>>>a = 4
>>>b = 5
>>>funcs_dict[s](a, b)
9
>>>funcs_dict['1'](4,5)
9
This is the same as doing this:
def do_func(s,a,b):
if s == '1':
add(a,b)
elif s == '2':
multiply(a,b)
elif s == '3':
subtract(a,b)
do_func('1', 4, 5)
So what is happening here?

Defined the three functions.

Created a dictionary
funcs_dict
of the functions. The keywords  in this case a number  and the values are the functions. it a Notice that there are no quotes around the function names. 
Then somehow (maybe an input from the user, or coming from another function, etc.) we got the value of
s
. And some values fora
andb
.
When we evaluate funcs_dict[s](a,b)
is when the cool stuff happens.

We go into the
funcs_dict
and pull the value of the given keyword. In this case, we get the value for the keyword'1'
, which is the functionadd
. 
And we add the parenthesis to make it a function call. In this case we also have some arguments to pass to the function  the
a
andb
. So what we are really doing isadd(a,b)
NOTE: We could have easily had a function that didn't use arguments, and then of course we wouldn't pass arguments; but it still needs the parentheses. It would look like this:funcs_dict[s]()
This can happen because ... Python functions are firstclass objects and can be stored as values in a dictionary. Just remember not to enclose them in quotes.