This question already has answers here:
Is there a difference between using a dict literal and a dict constructor?
(12 answers)
Closed 7 years ago.
Consider these two statements, which serve the same purpose:
tel = {'sape': 4139, 'jack': 4098}
and
tel = dict([('sape', 4139), ('jack', 4098)])
Why use "dict()" at all?
I am sure there is a reason, i just want to know it.
The reason for the existence of dict(...) is that all classes need to have a constructor. Furthermore, it may be helpful if the constructor is able to take in data in a different format.
In your example use case, there is no benefit in using dict, because you can control the format the data is in. But consider if you have the data already as pairs in a list, the dict constructor may be useful. This can happen e.g. when reading lines from a file.
map(dict,[[(1,2)]])
[{1: 2}]
map({},[[(1,2)]])
TypeError: 'dict' object is not callable
Related
This question already has answers here:
Why dict.get(key) instead of dict[key]?
(14 answers)
Closed 3 years ago.
monthConversions={
"Jan":"January", #key:value
"Feb":'February', #make sure keys are unique
'Mar':'March', #can also use integers
'Apr':"April",
'May':'May',
'Jun':'June',
}
print(monthConversions['Jan']) #gives value associated with the key
print(monthConversions.get('Luv','not a valid key'))
Hi, I am currently learning python through freebootcamp on youtube, and the person mentioned we can use the get function to pass in a value for a key that is not in the dictionary. I kind of understand that, but I fail to see what the use of this would be. I thought it would add Luv in when I put in
print(monthConversions['Luv'])
but it instead gives a error. What would be the purpose of the get function in this situation anyway? It feels like extra work and I don't get how it would be useful. Any explanations would be greatly appreciated, thank you.
It does not change the dict. The second argument is simply what get() returns when key is not found. If you don't specify it, it will simply return None.
This question already has answers here:
How do I create variable variables?
(17 answers)
How can I create multiple variables from a list of strings? [duplicate]
(2 answers)
generating variable names on fly in python [duplicate]
(6 answers)
Closed 3 years ago.
I have a ticker and I want to check a specific list of tickers to see if the ticker is found. If it is found, it will replace it.
The new tickers come from another data source and therefore do not know which specific list of tickers to check. In order to find that list, I can pass the lists name as a string but upon iterating the code (naturally) recognizes this as string as opposed to a list to iterate.
Is there a way to have the code/function recognize that the string is actually a specific list to be checked? In reading other questions, I know this may not be possible...in that case what is an alternative?
list_1=['A','B']
list_2=['C','D']
old_ticker='A'
new_ticker='E'
assigned_list='list_1'
def replace_ticker(old_ticker,new_ticker,list):
for ticker in list:
if new_ticker in list:
return
else:
list.append(new_ticker)
list.remove(old_ticker)
replace_ticker(old_ticker,new_ticker,assigned_list)
You key the needed lists by name in a dictionary:
ticker_directory = {
"list_1": list_1,
"list_2": list_2
}
Now you can accept the name and get the desired list as ticker_directory[assigned_list].
list_1=['A','B']
list_2=['C','D']
lists = {
'list_1':list_1,
'list_2':list_2
}
old_ticker='A'
new_ticker='E'
assigned_list='list_1'
def replace_ticker(old_ticker,new_ticker,list_name):
if old_ticker not in lists[list_name]:
return
else:
lists[list_name].append(new_ticker)
lists[list_name].remove(old_ticker)
replace_ticker(old_ticker,new_ticker,assigned_list)
print(lists[assigned_list])
This is the complete program from what i perceived.
#prune already answered this, I have just given the whole solution
There are at least two possibilities:
1 As noted in comments kind of overkill but possible:
Use eval() to evaluate string as python expressions more in the link:
https://thepythonguru.com/python-builtin-functions/eval/
For example:
list_name = 'list_1'
eval('{}.append(new_ticker)'.format(list_name))
2 Second
Using locals() a dictionary of locally scoped variables similiar to the other answers but without the need of creating the dict by hand which also requires the knowledge of all variables names.
list_name = 'list_1'
locals()[list_name].append(new_ticker)
This question already has answers here:
Python Dictionary DataStructure which method d[] or d.get()?
(5 answers)
Closed 8 years ago.
I have written a bit of code using
setting value
dic["key"] = "someval"
and fetching it the same way
print dic["key"]
then I discovered that an alternative way to fetch a dictionary value is to use
print dic.get("key")
I want all my code to use the same method, so should I rewrite all using dic.get("key") ?
If you have a flat dictionary and you want to add or modify a key/value pair, than the best way is to use a simple assignment:
h[key] = value
The get method is useful when you want to get a value from a dictionary for existing keys or use a default value for non-existing keys:
print h.get(key, "Does not exist!")
This question already has answers here:
Is it possible to keep the column order using csv.DictReader?
(6 answers)
Closed 9 years ago.
I have the need to deal using cvs.reader objects because of the order of the columnts must be preserved.
But, I would like also to have the csv.DictReader object for some other reasons.
Now, is there a simple posibility to convert them between each other without having re-reading the file?
I'd recommend you to use tablib instead.
ds = tablib.Dataset()
ds.csv = open(csvfile).read()
Then you can iter over ds as a sequence of rows, and ds.dict gives you a list of OrderedDict instances for each row. It's much easier if you also need to get it to other formats, or edit and save again as csv.
This question already has answers here:
Call a function with argument list in python
(6 answers)
Closed 9 years ago.
I am trying to write a python script that runs a bunch a function that are specified as a dictionary in JSON (this is user input). Here is what the JSON file contains:
{
"tests_to_run": {"find_holes": [],
"is_greater": [1.2]
}
}
find_holes and is_greater are two functions that I have written, the first taking 0, the second 1 parameter. The idea is to specify these parameters in the list associated with the function key in the JSON dictionary.
I am currently using this dictionary as follows:
functions_dict = {"find_holes": val.find_holes,
"is_monotonic": val.is_monotonic}
def run_functions(dict_tests_to_run):
for func, params in dict_tests_to_run.iteritems():
if params==[]:
functions_dict[func](time_series)
if len(params)==1:
functions_dict[func](time_series, params[0])
It seems a bit awkward to manually check for the length of the parameters, in particular since I am going to have to extend the loop to accomodate functions with up to at least 4 parameters. Is there any better way of doing this?
Thanks,
Anne
You can use the * operator to expand your list into function parameters. For example,
f(1,2,3)
is equivalent to:
a=[1,2,3]
f(*a)