python practice

check if dict has key

if key in dict

python 3.6 install package

after 3.4, pip is not standalone, to install a package, run
python -m pip install SomePackage

functools.partial

returns a function with partially providing the parameters
https://docs.python.org/2/howto/functional.html 

python threads pool

csvFiles = list(filter(isFile, os.listdir("d:/quotes")))
with  multiprocessing.Pool(multiprocessing.cpu_count() - 1) as p:
    p.map(loadCsv, csvFiles)
Here loadCsv is the function, csvFiles is the list to iterate
https://docs.python.org/3.6/library/multiprocessing.html

pickle

with open('filename', 'a') as f:
        pickle.dump(data, f)

with open('filename', 'rb') as f:
    x = pickle.load(f)

python prirnt current exception

import traceback
traceback.print_exc()

zip two arrays into tuples

let's say you have two arrays [-1, 0, 1, 2] and [7,8,9,10]
you want to merge them into tuples like this [(-1, 7), (0, 8), (1, 9), (2, 10)]
list(zip(array1, array2))

array filter

result = [a for a in A if a not in subset_of_A]

python function argument

http://detailfocused.blogspot.ca/2017/07/python-function-parameters.html

define function inside function

def method_a(arg):

    def method_b(arg):
        return some_data

    some_data = method_b

python class and inheritance

http://www.cnblogs.com/feeland/p/4419121.html
https://docs.python.org/3/tutorial/classes.html

Generator and yield

http://detailfocused.blogspot.ca/2017/08/python-generator-and-yield.html

Set

Intersection - to find the same items
Difference - to find the different items
create sets using the new notation

a_set = {'red', 'blue', 'green'}
print(type(a_set))
# Output: type 'set'

>>> a = {1,1,2,3}
>>> a
{1, 2, 3}

http://book.pythontips.com/en/latest/set_-_data_structure.html

Decorators

Decorator is to add a wrap to the target function. It is like the interceptor in spring framework.
When use pymongo to query mongodb, it can return Cursor or list if it is sorted. This inconsistency is quite annoying to the client code .
I added the decorator "wrapReturnToList" to make all return to be List.

def wrapReturnToList(f):
    @wraps(f)
    def decorated(*args, **kwargs):
        return list(f(*args, **kwargs))
    return decorated

@wrapReturnToList
def getAllActiveSymbols():
    return stockDb.symbol.find({ "Status": { "$ne": SYMBOL_INACTIVE } })

Without the annotation, it returns Cursor. With the annotation, it returns List.
http://book.pythontips.com/en/latest/decorators.html

zipfile

    zf = zipfile.ZipFile(dir + zipFileName, mode='a')
    try:
        zf.write(toZipFileFullPat, os.path.basename(toZipFileFullPath))
    finally:
        zf.close()

os.path.basename(toZipFileFullPat) is to get the file name from the full path, so that it won't zip the dir into the zip file.

defaultdict

defaultdict does not need to check whether a key is present or not.

from collections import defaultdict

colours = (
    ('Yasoob', 'Yellow'),
    ('Ali', 'Blue'),
    ('Arham', 'Green'),
    ('Ali', 'Black'),
    ('Yasoob', 'Red'),
    ('Ahmed', 'Silver'),
)

favourite_colours = defaultdict(list)

for name, colour in colours:
    favourite_colours[name].append(colour) #favourite_colours[name] may not exist, dict throws error

print(favourite_colours)

# output
# defaultdict(,
#    {'Arham': ['Green'],
#     'Yasoob': ['Yellow', 'Red'],
#     'Ahmed': ['Silver'],
#     'Ali': ['Blue', 'Black']
# })

http://book.pythontips.com/en/latest/collections.html#defaultdict

deque

Double end queue, can pop or extend items from either end.
http://book.pythontips.com/en/latest/collections.html#deque