I'm trying to create a function (without using a built in function) that turns this
([f,u,n],[1,2,3,4])
into this [(f,1),(u,2),(n,3)]
Basically taking the first element of both lists and turning them into a tuple, then the second element of both lists, etc.
I have this so far:
>> def zipup(lista,listb):
>> for x in range(len(lista)):
>>for y in range(len(listb)):
>>return [(lista[x],listb[y])]
But I keep getting this: [(f,1)]
Help?!
The error message is saying that the variable i is not defined, it is not even in the code snippet that you showed ... that a look at the line that the error shows.
As for the zip, take a look at the docs for the standard library for the izip, there is a equivalent python implementation that you can use as a guide.
Related
I came across an issue and I would love to get your help.
I am working with a certain codebase at work, and came across a function which returns 2 values, let's call this function my_func.
In some other function, my_func's output is being unpacked, for example:
a,b = my_func()
The thing is, I want to expand my_func to return 3 values instead of 2, however my_func is sort of an "interface function" which means that it is implemented by a lot of classes and I'm trying to avoid going to each of these classes and extend it's implementation directly.
I was wondering if there's something I can do to get the following results:
a,b,c = my_func()
without getting any unpacking exception. I would like if possible that c will get the value None from the "old implementation" classes.
Is this a good / bad practice? would love to get your opinion and help.
Short version:
How do you store functions in a list and only have them be executed when they are called using their index position in the list?
Long Version:
So I am writing a program that rolls a user-chosen number of six-sided dice, stores the results in a list and then organizes the results/ data in a dictionary.
After the data is gathered the program gives the user options from 0-2 to choose from and asks the user to type a number corresponding to the option they want.
After this input by the user, a variable, lets say TT, is assigned to it. I want the program to use TT to identify which function to run that is contained within a list called "Executable_options" by using TT as the index posistion of this function within the list.
The problem I am having is that I have to have the list that contains the functions on a line after the functions have been defined and when I initialize the list it goes through and executes all functions within it in order when I don't want it to. I just want them to be in the list for calling at a later date.
I tried to initialise the list without any functions in and then append the functions individually, but every time a function is appened to the list it is also executed.
def results():
def Rolling_thunder():
def roll_again():
The functions contains things, but is unnecessary to show for the question at hand
Executable_options = []
Executable_options.append(results())
Executable_options.append(Rolling_thunder())
Executable_options.append(roll_again)
options = len(Executable_options)
I am relatively new to Python so I am still getting my head around it. I have tried searching for the answer to this on existing posts, but couldn't find anything so I assume I am just using the wrong key words in my search.
Thank you very much for taking the time to read this and for the answers provided.
Edit: Code now works
The () on the end of the function name calls it - i.e. results() is the call to the results method.
Simply append to the list without the call - i.e:
Executable_options.append(results)
You can then call it by doing e.g.:
Executable_options[0]()
as per your given data the code will look like this:
def results():
def Rolling_thunder():
def roll_again():
Executable_options = []
Executable_options.append(results)
Executable_options.append(Rolling_thunder)
Executable_options.append(roll_again)
for i in range(0,len(Executable_options)):
Executable_options[i]()
this will work for you.
In my recently migrated from 2 to 3 Python code I have
list(reversed(zip(*positions)))
which generates the error
TypeError: 'zip' object is not reversible
I can fix this by changing the problematic code to
list(reversed(list(zip(*positions))))
but this seems like the wrong way to go about it.
What is the correct way to revers a zip in Python 3?
reversed is used to iterate on the list. It doesn't create a list on purpose, because it's often used just to iterate backwards on elements, not to create lists.
That's why you have to use list on it to create a list. And it needs a sequence to be able to get to the last element directly so you have to do list(zip()) in python 3.
Maybe you could shorten
list(reversed(list(zip(*positions))))
to
list(zip(*positions))[::-1]
it creates a list directly without the need for reverse so it's probably slightly faster too.
In boto3 there's a function:
ec2.instances.filter()
The documentation:
http://boto3.readthedocs.org/en/latest/reference/services/ec2.html#instance
Say it returns a list(ec2.Instance) I wish...
when I try printing the return I get this:
ec2.instancesCollection(ec2.ServiceResource(), ec2.Instance)
I've tried searching for any mention of an ec2.instanceCollection, but the only thing I found was something similar for ruby.
I'd like to iterate through this instanceCollection so I can see how big it is, what machines are present and things like that.
Problem is I have no idea how it works, and when it's empty iteration doesn't work at all(It throws an error)
The filter method does not return a list, it returns an iterable. This is basically a Python generator that will produce the desired results on demand in an efficient way.
You can use this iterator in a loop like this:
for instance in ec2.instances.filter():
# do something with instance
or if you really want a list you can turn the iterator into a list with:
instances = list(ec2.instances.filter())
I'm adding this answer because 5 years later I had the same question and went round in circles trying to find the answer.
First off, the return type in the documentation is wrong (still). As you say, it states that the return type is: list(ec2.Instance)
where it should be:ec2.instancesCollection.
At the time of writing there's an open issue in github covering this - https://github.com/boto/boto3/issues/2000.
When you call the filter method a ResourceCollection is created for the particular type of resource against which you called the method. In this case the resource type is instance which gives an instancesCollection. You can see the code for the ResourceCollection superclass of instancesCollection here:
https://github.com/boto/boto3/blob/develop/boto3/resources/collection.py
The documentation here gives an overview of the collections: https://boto3.amazonaws.com/v1/documentation/api/latest/guide/collections.html
To get to how to use it and actually answer your question, what I did was to turn the iterator into a list and iterate over the list if the size is > 0.
testList = list(ec2.instances.filter(Filters=filters))
if len(testList) > 0;
for item in testList;
.
.
.
This may well not be the best way of doing it but it worked for me.
Here are two examples:
sum(list(map(lambda x:x,range(10))))
and
sum(range(10))
The second example does not require a list(), but the first one does. Why?
How do I know when is list() a necessity? Similarly using list() for min() and max().
I am running python 3.3.5 with ipython 2.2.0. Here is what I see:
print(sum) results in <built-in function sum> from python console and <function sum at 0x7f965257eb00> from ipythonNotebook. Looks like an issue with hidden imports in notebook.
Neither of the examples require the use of list. The sum builtin function works with any iterable, so converting the result of map to a list isn't necessary.
Just in case, make sure you are indeed using the builtin sum function. Doing something like from numpy import * would override that. (you can simply print sum and see what you get).
I guess the 1st one just enforces and expects the output of the map function to be a list because if there are multiple arguments, map() returns a list consisting of tuples containing the corresponding items from all iterables.
But either way base on your example, it would still work.