This question already has answers here:
What does "list comprehension" and similar mean? How does it work and how can I use it?
(5 answers)
Closed 3 years ago.
I have scoured the internet and I cannot find any reference to this type of for loop:
variable = [(item["attribute1"], item["attribute2]") for item in piece_of_json_data]
I am using this to update wtform's SelecField choices:
form.SelectField.choices = variable
but I can only get it to work if I replace one of the attributes in parenthesis with a static number:
variable = [(1, item["attribute2"]) for item in piece_of_json_data]
but that sets the value of the option field to "1", when I need the option values to be one of the attributes as a string.
Does this create a dict? a tuple? is there some kind of terminology for this that I can use to find documentation?
Thanks to the comments, I now understand that I am using list comprehension to create a tuple. The tuple works fine with both string and integer values. My issue has to do with .choices not accepting a string.
I found that my only problem is that I had coerce set to int on my selectfield, so naturally it wanted an integer.
Related
This question already has answers here:
What do ellipsis [...] mean in a list?
(5 answers)
Closed 2 years ago.
I've run into this unseen list object, when I tried to play with list append, and I've searched hard but unable to find much information. So this is what's happening:
L = ['dinosaur']
L.append(('theropoda', L))
print(L)
# ['dinosaur', ('theropoda', [...])]
Questions - what's the [...] means here? Thanks.
As mentioned in the comments, Python will not attempt to include a circular/recursive reference in the representation of a list.
It would appear that the __repr__ function (also used by lists to create the string for printing) is implemented via reprlib with recursive support. Without it, you would end up with a RecursionError as to output the list, Python must include the nested version of the list, which also needs the nested version, and so on. Instead, it outputs the special value of ... which indicates to you that it is a recursive reference.
This question already has answers here:
What is the purpose of the single underscore "_" variable in Python?
(5 answers)
Trying to understand Python loop using underscore and input
(4 answers)
Closed 4 years ago.
I was check a solution on hacker rank where i was solving a question asking to print the name of the person with the second highest score from an input which has to be converted to a nested list first .
I understood all the logic in the code and most part of the code but why the Underscore ( _ ) in the for loop .Please explain me the code if there is a different concept .
marksheet = []
for _ in range(0,int(input())):
marksheet.append([input(), float(input())])
second_highest = sorted(list(set([marks for name, marks in marksheet])))[1]
print('\n'.join([a for a,b in sorted(marksheet) if b == second_highest]))
It's a Pythonic convention to use underscore as a variable name when the returning value from a function, a generator or a tuple is meant to be discarded.
In your example, the code inside the for loop does not make any use of the values generated by range(0,int(input())), so using an underscore makes sense as it makes it obvious that the loop does not intend to make use of it.
This question already has answers here:
Split string in Lua?
(18 answers)
Closed 9 years ago.
I have one string, that is = s = "Pedro Martinez, Defense"
I want to split the string before the comma and after the comma, store those cuts on 2 variables, for example:
I think that I need to use the string.gmatch function or string.sub
How can I do that?
The accepted answer at How to convert GPS coordinates to decimal in Lua? shows how to use string.match and patterns. Applying the same techniques as mentioned there, you could use
local name, expertise = string.match(s, "(.*),%s*(%a*)")
which would lead to name being "Pedro Martinez" and expertise being "Defense".
This question already has answers here:
Find file name from full file path
(4 answers)
Closed 9 years ago.
This should be dead simple, but I can't figure it out. I have a string that represents a path name to a file, ie
"C:/blah/this/whatever/filename"
And I need to extract the filename (programatically). I am trying to use strsplit (or str_split) and, it is easy enough to split the string by '/' but.. I cannot fathom how to actually extract one of the values in the returned vector, or determine how many elements are even in the vector (as that may very for my application). Using length (oddly enough, to my mind) does not help. Help?
Taken from: Find file name from full file path
basename("C:/some_dir/a")
> [1] "a"
dirname("C:/some_dir/a")
>[1] "C:/some_dir"
Although I think the above approach is much better, you can also use the str_split approach - which I really only mention to show how to select the last elements from a list using lapply.
example <- c("C:/some_dir/a","C:/some_dir/sdfs/a","C:/some_dir/asdf/asdf/a")
example.split <- strsplit(example,"/")
files <- unlist(lapply(example.split, tail , 1 ))
Don't need str_split:
sub( "^.+/(.+)$", "\\1", "C:/blah/this/whatever/filename" )
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Annoying PHP error: “Strict Standards: Only variables should be passed by reference in”
I have this line of code,
$extension=end(explode(".", $srcName));
when I fun my function I get
PHP Strict Standards: Only variables should be passed by reference in
I am not sure how to solve this
The function end() requires a variable to be passed-by-reference and passing the return-value of a function doesn't acheive this. You'll need to use two lines to accomplish this:
$exploded = explode(".", $srcName);
$extension = end($exploded);
If you're simply trying to get a file-extension, you could also use substr() and strrpos() to do it in one line:
$extension = substr($srcName, strrpos($srcName, '.'));
Or, if you know the number of .'s that appear in the string, say it's only 1, you can use list() (but this won't work if there is a dynamic number of .'s:
list(,$extension) = explode('.', $srcName);