[SOLVED] Python

Sefyir · 03-17-2017, 11:18 PM

I'm new to python and attempting to learn python3.
I have a book that instructs

Quote:

Write a function called make_great() that modifies the list of magicians by adding the phrase the Great to each magicians name

I have written the below, but I can't make sense why after leaving the function, the list becomes []

Code:

#!/usr/bin/python3
magicians = ['useryeah', 'usercool', 'userno', 'duck']                          
print(magicians)                                                                
                                                                                
def make_great(magicians_list):                                                 
  great_magicians_list = []                                                     
  while magicians_list:                                                         
    great_magicians_name = 'the Great ' + magicians_list.pop()                  
    great_magicians_list.append(great_magicians_name.title())                   
    magicians = great_magicians_list                                            
  print(magicians)                                                              
                                                                                
                                                                                
make_great(magicians)                                                           
print(magicians)

I get this output

Code:

['useryeah', 'usercool', 'userno', 'duck']
['The Great Duck', 'The Great Userno', 'The Great Usercool', 'The Great Useryeah']
[]

but I expect

Code:

['useryeah', 'usercool', 'userno', 'duck']
['The Great Duck', 'The Great Userno', 'The Great Usercool', 'The Great Useryeah']
['The Great Duck', 'The Great Userno', 'The Great Usercool', 'The Great Useryeah']

dijetlo · 03-18-2017, 01:01 AM

You globally declared the variable and passed it to a function, which has it's own scope. You then referenced the variable in a print statement in the global scope without returning a value for the variable from the function scope.
It has no value because no value was passed back to the global scope from the functions scope after it was modified.
Local scopes can inherit from enclosing scopes, however they cannot set a value outside their own scope unless you do so explicitly in the enclosing scope.
Confused? No worries, it makes better sense if you read the code...

Code:

#!/usr/bin/python3
magicians = ['useryeah', 'usercool', 'userno', 'duck']                          
print(magicians)                                                                
                                                                                
def make_great(magicians_list):                                                 
  great_magicians_list = []                                                     
  while magicians_list:                                                         
    great_magicians_name = 'the Great ' + magicians_list.pop()                  
    great_magicians_list.append(great_magicians_name.title())                   
    magicians = great_magicians_list 
  print magicians                                         
  return magicians                                                              
                                                                                
                                                                                
magicians = make_great(magicians)
print(magicians)

Produces:

Quote:

bash-4.4$ python lq.py
['useryeah', 'usercool', 'userno', 'duck']
['The Great Duck', 'The Great Userno', 'The Great Usercool', 'The Great Useryeah']
['The Great Duck', 'The Great Userno', 'The Great Usercool', 'The Great Useryeah']

This, on the other hand

Code:

...
print(make_great(magicians))
print(magicians)

Produces

Quote:

['useryeah', 'usercool', 'userno', 'duck']
['The Great Duck', 'The Great Userno', 'The Great Usercool', 'The Great Useryeah']
[]

Because in the second example I didn't specifically set the global variable magicians to the return value, I just printed it.
Scoping can be a rough spot, everyone has to puzzle it out.
Last suggestion:
A better way to do this is list comprehension

Code:

#!/usr/bin/python3
magicians = ['useryeah', 'usercool', 'userno', 'duck']                          
print(magicians)                                                                
                                                                                
def make_great():                                                 
  return [ 'The Great ' + x for x in magicians]                                                               
                                                                                                                                                               
magicians = make_great()
print(magicians)

Just remember, Python is extremely cool, and you are cool for learning it

Sefyir · 03-18-2017, 09:34 PM

This helped tremendously!
I am reminded of bash and subprocesses when it comes to scoping.
For example,

Code:

$ a=hello; (a=goodbye); echo "$a"
hello

So I presume a analogy of return is it "escapes" the current confined scope of a function into the global scope?
I had not read yet about return (I think) but I'll test around with it

Also, I had read about list comprehension, but this made me realize how it could be relevant
I will have to test with it more too.

Thank you!

dijetlo · 03-18-2017, 09:52 PM

Quote:

Also, I had read about list comprehension, but this made me realize how it could be relevant
I will have to test with it more too.

Let me know if I can be of further help.

dugan · 03-19-2017, 03:12 PM

I know this wasn't the question, but I wanted to comment on your loops.

The Pythonic way to get that modified list would be with a list comprehension:

Code:

great_magicians = [m + " the great" for m in magicians]

Or with Python 3.6:

Code:

great_magicians = [f"{m} the great" for m in magicians]

A foreach loop is another common option:

Code:

great_magicians = []
for m in magicians:
    great_magicians_list.append(m + " the great")

A map function is one that I don't see much (list comprehensions are better), but it's also supported:

Code:

great_magicians = list(map(lambda m: m + ' the great', magicians))

dijetlo · 03-19-2017, 09:17 PM

Eh, Sefyirs while loop ran without issue, which is pythonic enough for a beginner. His problem was scoping but he seems to have gotten a handle on that. Let's not confuse the young Jedi unnecessarily, he's just putting his light sabre together. He'll come to the pythonic force on his own, given time (assuming he doesn't choose the dark side, aka Ruby)

We should probably start a 'Stupid Python Tricks' thread if we want to get into optimizations and machine learning atomics.
On that note, Dugan, do you have any background with anaconda (the scipy environment) and if you do, can you recommend a tutorial ? I'm just getting started with it and it looks like Guidos swiss army knife (a little confusing for a mere mortal)

dugan · 03-20-2017, 09:14 AM

Quote:

Originally Posted by dijetlo

On that note, Dugan, do you have any background with anaconda (the scipy environment) and if you do, can you recommend a tutorial ?

I don't, sorry.