a.資料科學 - Python 速成班(進階)

# a.資料科學 - Python 速成班(進階) ###### tags: `Data Sicnce From Scratch` > **#IPython有個奇妙的功能 %paste，它可以正確貼上任何你放在剪貼簿中的程式碼** ## 1.函式 (Function) * **頭等(First-class)函式** ```python=+ #Python 的函式屬於一等(First-class)函式，也就是說，我們可以把函式指定給某個變數，然後把它傳進某個函式中 def double(x): return x * 2 def apply_to_one(f): """給予傳進來的函式「1」的參數""" return f(1) my_double = double result = apply_to_one(my_double) print(result) ``` > ```2``` ## * **短匿名函式(或lambda函式)** ```python=+ y = apply_to_one(lambda x:x+4) print(y) ``` > ```5``` ## * **把lambda指定給變數，不推薦** ```python=+ another_double = lambda x:x*2 print(another_double(2)) def another_double(x): """建議改用這種作法""" return x*2 print(another_double(2)) ``` > ```4``` > ```4``` ## * **指定函式內參數名稱** ```python=+ def full_name(first = "What's-his-name",last = "Something"): return first + " " + last full_name("Joel","Grus") full_name("Joel") full_name(last = "Grus") ``` > ```What's-his-name Grus``` ## 2.字串 (String) * **跳脫字元** ```python=+ tab_string = "\t" print(tab_string,len(tab_string)) ``` > ``` 1``` ## * **建立原始字串** ```python=+ r_tab_string = r"\t" #用「r」建立raw(原始)-string print(r_tab_string,len(r_tab_string)) ``` > ```\t 2``` ## 3.例外處理 (Exception) * **簡單的例外處理結構** ```python=+ try: print(0 / 0) except ZeroDivisionError: print("cannot divide by zero!!") else: print("unexpected error!!") finally: print(">>process end") ``` > ```cannot divide by zero!!``` > ```>>process end``` ## 4.列表 (List) * **列表的新增** ```python=+ x = [1,2,3] x.extend([4,5,6]) #會更改列表本身 print(x) x = [1,2,3] y = x + [4,5,6] #如不想更改列表本身可用「+」 y ``` > ```[1, 2, 3, 4, 5, 6]``` > ```[1, 2, 3, 4, 5, 6]``` ## 5.元組 (Tuple) > **#例外處理(Exception):#修改元組內容 - TypeError** * **例外處理** ```python=+ my_list = [1,2] my_tuple = (1,2) other_tuple = 3,4 my_list[1] = 3 try: my_tuple[1] = 3 except TypeError: print("cannot modify tuple's content!") ``` > ```cannot modify tuple's content!``` ## * **函式回傳多個值** ```python=+ def sum_and_product(x,y): return (x+y),(x*y) #依舊是回傳一個值，因為統一被打包成一個Tuple sp = sum_and_product(2,3) s,p = sum_and_product(5,10) print("sp =",sp) print("s =",s,"p =",p) ``` > ```sp = (5, 6)``` > ```s = 15 p = 50``` ## * **多重賦值** ```python=+ #元組和列表都可使用多重賦值(multiple assignment) x,y = 1,2 x,y = y,x print("x =",x) print("y =",y) ``` > ```sp = (5, 6)``` > ```s = 15 p = 50``` ## 6.字典 (Dict) > **#例外處理(Exception):#不存在的鍵值來取值 - KeyError** * **例外處理** ```python=+ grades = {"Joel":80, "Tim":95} try: kate_grade = grades["Kate"] except KeyError: print("cannot found assign key!!") ``` > ```cannot found assign key!!``` ## * **字典的取值方法** ```python=+ grades = {"Joel":80, "Tim":95} joels_grade = grades.get('Joel',0) print(joels_grade) kates_grade = grades.get("Kate",0) print(kates_grade) no_ones_grade = grades.get("No One") #get方法不會產生例外 print(no_ones_grade) ``` ## * **字典的檢查方法** ```python=+ grades = {"Joel":80, "Tim":95} grades_keys = grades.keys() grades_values = grades.values() #檢查鍵值 print("Joel" in grades_keys) print("Kate" in grades) #推薦 #檢查對應值(唯一方法) print(95 in grades_values) ``` > ```True``` > ```False``` > ```True``` > **#(注意!)字典的鍵值必須是「可湊化的(hashable)」;尤其注意絕不能用列表(list)當作鍵值 #需要多維的鍵值，就應該使用元組，或是把鍵值轉換成一個字串!!** ## 7.Defaultdict * **計算整份文件的單詞數量(利用例外處理:先做再說，錯了再補救)** ```python=+ word_counts = {} def document_counts(document): for word in document: try: word_counts[word] += 1 except: word_counts[word] = 1 print(word_counts) ``` ## * **計算整份文件的單詞數量(優雅的作法)** ```python=+ word_counts = {} def document_counts(document): for word in document: previous_count = word_counts.get(word,0) word_counts[word] = previous_count + 1 ``` ## * **最乾淨俐落的作法 - defaultdict** ```python=+ from collections import defaultdict word_counts = defaultdict(int) #使用前必須先指定預設 - int()會生成「0」 def document_counts(document): for word in document: word_counts[word] += 1 ``` ## * **(續)最乾淨俐落的作法 - defaultdict** ```python=+ dd_list = defaultdict(list) dd_list[2].append(1) #先建立出空陣列{2:[]} => 再append{2:[1]} print(dict(dd_list)) #要轉換成dict來使用、查看 dd_dict = defaultdict(dict) dd_dict["Joel"]["City"] = "Seattle" #先建立出空字典{'Joel':{}} => 再{'Joel': {'City': 'Seattle'}} print(dict(dd_dict)) #要轉換成dict來使用、查看 dd_pair = defaultdict(lambda : [0,0]) dd_pair[2][1] = 1 print(dict(dd_pair)) ``` > ```{2: [1]}``` > ```{'Joel': {'City': 'Seattle'}}``` > ```{2: [0, 1]}``` ## 8.計數器 (Counter) * **建立一個計數器** ```python=+ from collections import Counter c = Counter([0,1,2,0]) # c = {'0':2,'1':1,'2':1} c ``` > ```Counter({0: 2, 1: 1, 2: 1})``` ## * **計數最常出現的單詞(word)** ```python=+ def document_counts(document): word_counts = Counter(document)#document 是由許多單詞(word)構成的一個列表 for word,count in word_counts.most_common(10): #輸出前10個最常用的單詞(word)，及其出現次數(count) print(word,count) ``` ## 9.集合 (Set) > **#針對set進行in操作，速度非常快(因為重複的元素會被刪除)，因此檢查次數減少** ## 10.控制流程 (Process Control) > **#if-then-else#while,for,in,continue,break** ## 11.真(True)與假(False) * **針對 'None' 的真假判斷** ```python=+ x = None assert x == None #True assert x is None #推薦寫法 #assert x is False #若執行此程式碼，則產生例外 ``` ## * **Python 的邏輯判斷** ```python=+ s = 'string' first_char = s and s[0] # True and True -> 傳回s「0」 print(first_char) s = '' first_char = s and s[0] # False and True -> 傳回s first_char ``` ## * **(!)(續)Python 的邏輯判斷** ```python=+ #all()函式 -> 其一為False，則結果為False，否則True a = all([True,1,{3}]) #True b = all([True,1,{}]) #False #any()函式 -> 其一為True，則結果為True，否則False c = any([True,1,{}]) #True d = any([None,0,{}]) #False e = all([]) #Ture，因為沒有代表「False」的元素 f = any([]) #False，因為沒有代表「True」的元素 print(a,b,c,d,e,f) ``` ## 11.排序 (Sort) * **指定函式條件 (Ex.abs) 進行排序** ```python=+ x = [-4,1,-2,3] x.sort(key = abs,reverse = True) x ``` > ```[-4, 3, -2, 1]``` ## * **自訂函式條件進行排序** ```python=+ def return_Length(ele): return len(ele) x = ['Lion','Frog','Dog','Elephant'] y = sorted(x,key = return_Length,reverse = True) y ``` > ```['Elephant', 'Lion', 'Frog', 'Dog']``` ## * **(續)自訂函式條件進行排序** ```python=+ word_counts = {'Lion':3,'Rabbit':5,'Cat':10,"Dog":6} sort_by_length = sorted(word_counts.items(),key = lambda word_and_counts:len(word_and_counts[0])) sort_by_counts = sorted(word_counts.items(),key = lambda word_and_counts:word_and_counts[1]) print(sort_by_length) #依照字串長度排列 print(sort_by_counts) #依照出現次數排列 ``` > ```[('Cat', 10), ('Dog', 6), ('Lion', 3), ('Rabbit', 5)]``` > ```[('Lion', 3), ('Rabbit', 5), ('Dog', 6), ('Cat', 10)]``` ## 12.解析式列表 (List Comprehension) * **解析式列表套入條件判斷** ```python=+ evens = [x for x in range(5) if x%2 == 0] evens ``` > ```[0, 2, 4]``` ## * **解析式字典與解析式集合** ```python=+ square_dict = {x:x*x for x in range(5)} square_dict square_set = {x*x for x in [-2,-1,0,1,2]} square_set ``` > ```{0: 0, 1: 1, 2: 4, 3: 9, 4: 16}``` > ```{0, 1, 4}``` ## * **<!>解析式列表套入多重迴圈** ```python=+ #可有多個for迴圈 pairs = [(i,j) for i in range(5) for j in range(2)] pairs ``` > ```[(0, 0),(0, 1),(1, 0),(1, 1),(2, 0),(2, 1),(3, 0),(3, 1),(4, 0),(4, 1)]``` ## * **<!>(續)解析式列表套入多重迴圈** ```python=+ #後面的迴圈可以使用前面的結果 increasing_pairs = [(x,y) for x in range(5) for y in range(x,x+1)] increasing_pairs ``` > ```[(0, 0), (1, 1), (2, 2), (3, 3), (4, 4)]``` ## 13.斷言 (Assert) * **assert(斷言) 若條件不成立則產生例外** ```python=+ def smallest_items(xs): assert xs,"空項目不會有最小項" #AssertionError: 空項目不會有最小項(自訂的輸出內容) return min(xs) assert smallest_items([5,10,15,20]) == 5 assert smallest_items([]) ``` > ```AssertionError: 空項目不會有最小項``` ## 14.zip(參數壓合) 與 *(參數拆分) * **參數壓合與參數拆分** ```python=+ list1 = ['a','b','c'] list2 = [1,2,3] #zip參數壓合 x = [ele for ele in zip(list1,list2)] print(x) #zip參數拆分 alpha,num = zip(*x) print(alpha,num) ``` > ```[('a', 1), ('b', 2), ('c', 3)]``` > ```('a', 'b', 'c') (1, 2, 3)``` ## * **(續)參數壓合與參數拆分** ```python=+ def add(x,y):return x+y result = add(*[2,3]) #把list[2,3] 拆分成 2,3 result ``` > ```5``` ## 15.args 與 kwargs * **將函式做為高階函式的輸入** ```python=+ def doubler(f): #定義一個新函式g，它會參照到f的結果 def g(x): return 2*f(x) return g #這裡要return g(注意!!) ``` ## * **(續)將函式做為高階函式的輸入測試** ```python=+ def f1(x): return x+1 d = doubler(f1) assert d(3) == 8,"(3+1) * 2 應等於 8" assert d(-1) == 0,"(-1+1)*2 應等於 0" #當函式有多個參數就會出問題 #若指定「assert d(1,2)」會出錯，因為d只能接收一個值 ``` ## * **args 與 kwargs 的簡單結構** ```python=+ def magic(*args,**kwargs): print("unnamed args:",args) print("keyword args:",kwargs) magic(1,2,key = 'word',key2 = 'word2') ``` > ```unnamed args: (1, 2)``` > ```keyword args: {'key': 'word', 'key2': 'word2'}``` ## * **(續)args 與 kwargs 的簡單結構** ```python=+ def other_way_magic(x,y,z): return x+y+z x_y_list = [1,2] z_dict = {"z":3} #利用*和**為函式提供參數值 assert other_way_magic(*x_y_list,**z_dict) == 6,"1+2+3 應等於 6" ``` ## 時間戳記 > [name=ZEOxO][time=Tue, Sep 8, 2020 16:15 PM][color=#907bf7]