Functions and Getting Help

在本課中，我們將討論函數(shù)：調(diào)用它們，定義它們，并使用Python的內(nèi)置文檔查找它們。
在某些語(yǔ)言中，定義函數(shù)必須要有特定的參數(shù)，每個(gè)參數(shù)都具有特定類型。 Python函數(shù)允許更靈活。 print函數(shù)就是一個(gè)很好的例子：

[1]

print("The print function takes an input and prints it to the screen.") print("Each call to print starts on a new line.") print("You'll often call print with strings, but you can pass any kind of value. For example, a number:") print(2 + 2) print("If print is called with multiple arguments...", "it joins them","(with spaces in between)", "before printing.") print('But', 'this', 'is', 'configurable', sep='!...') print() print("^^^ print can also be called with no arguments to print a blank line.") The print function takes an input and prints it to the screen. Each call to print starts on a new line. You'll often call print with strings, but you can pass any kind of value. For example, a number: 4 If print is called with multiple arguments... it joins them (with spaces in between) before printing. But!...this!...is!...configurable^^^ print can also be called with no arguments to print a blank line.

"What does this function do again?"

在前面的章節(jié)中我已經(jīng)介紹了abs函數(shù)，但是如果你忘了它的作用怎么辦？

help()函數(shù)可能是你學(xué)習(xí)的最重要的Python函數(shù)。 如果你能記住如何使用help()，那么你就掌握了解Python中任何其他函數(shù)。

[2]

help(abs) Help on built-in function abs in module builtins:abs(x, /)Return the absolute value of the argument.

應(yīng)用于函數(shù)時(shí)，help()顯示...

• ???? 該函數(shù)的頭部為abs（x，/）。 在這種情況下，這告訴我們abs（）采用單個(gè)參數(shù)x。 （正斜杠并不重要，但如果你很好奇，你可以在這里閱讀）
• ???? 關(guān)于該功能的簡(jiǎn)要英文描述。

常見(jiàn)的陷阱：當(dāng)你查找函數(shù)時(shí)，記得傳入函數(shù)本身的名稱，而不是調(diào)用該函數(shù)的結(jié)果。

如果我們?cè)谡{(diào)用函數(shù)abs（）時(shí)調(diào)用幫助會(huì)發(fā)生什么？ 看看下面這個(gè)例子：

[3]

help(abs(-2)) Help on int object:class int(object)| int(x=0) -> integer| int(x, base=10) -> integer| | Convert a number or string to an integer, or return 0 if no arguments| are given. If x is a number, return x.__int__(). For floating point| numbers, this truncates towards zero.| | If x is not a number or if base is given, then x must be a string,| bytes, or bytearray instance representing an integer literal in the| given base. The literal can be preceded by '+' or '-' and be surrounded| by whitespace. The base defaults to 10. Valid bases are 0 and 2-36.| Base 0 means to interpret the base from the string as an integer literal.| >>> int('0b100', base=0)| 4| | Methods defined here:| | __abs__(self, /)| abs(self)| | __add__(self, value, /)| Return self+value.| | __and__(self, value, /)| Return self&value.| | __bool__(self, /)| self != 0| | __ceil__(...)| Ceiling of an Integral returns itself.| | __divmod__(self, value, /)| Return divmod(self, value).| | __eq__(self, value, /)| Return self==value.| | __float__(self, /)| float(self)| | __floor__(...)| Flooring an Integral returns itself.| | __floordiv__(self, value, /)| Return self//value.| | __format__(...)| default object formatter| | __ge__(self, value, /)| Return self>=value.| | __getattribute__(self, name, /)| Return getattr(self, name).| | __getnewargs__(...)| | __gt__(self, value, /)| Return self>value.| | __hash__(self, /)| Return hash(self).| | __index__(self, /)| Return self converted to an integer, if self is suitable for use as an index into a list.| | __int__(self, /)| int(self)| | __invert__(self, /)| ~self| | __le__(self, value, /)| Return self<=value.| | __lshift__(self, value, /)| Return self<<value.| | __lt__(self, value, /)| Return self<value.| | __mod__(self, value, /)| Return self%value.| | __mul__(self, value, /)| Return self*value.| | __ne__(self, value, /)| Return self!=value.| | __neg__(self, /)| -self| | __new__(*args, **kwargs) from builtins.type| Create and return a new object. See help(type) for accurate signature.| | __or__(self, value, /)| Return self|value.| | __pos__(self, /)| +self| | __pow__(self, value, mod=None, /)| Return pow(self, value, mod).| | __radd__(self, value, /)| Return value+self.| | __rand__(self, value, /)| Return value&self.| | __rdivmod__(self, value, /)| Return divmod(value, self).| | __repr__(self, /)| Return repr(self).| | __rfloordiv__(self, value, /)| Return value//self.| | __rlshift__(self, value, /)| Return value<<self.| | __rmod__(self, value, /)| Return value%self.| | __rmul__(self, value, /)| Return value*self.| | __ror__(self, value, /)| Return value|self.| | __round__(...)| Rounding an Integral returns itself.| Rounding with an ndigits argument also returns an integer.| | __rpow__(self, value, mod=None, /)| Return pow(value, self, mod).| | __rrshift__(self, value, /)| Return value>>self.| | __rshift__(self, value, /)| Return self>>value.| | __rsub__(self, value, /)| Return value-self.| | __rtruediv__(self, value, /)| Return value/self.| | __rxor__(self, value, /)| Return value^self.| | __sizeof__(...)| Returns size in memory, in bytes| | __str__(self, /)| Return str(self).| | __sub__(self, value, /)| Return self-value.| | __truediv__(self, value, /)| Return self/value.| | __trunc__(...)| Truncating an Integral returns itself.| | __xor__(self, value, /)| Return self^value.| | bit_length(...)| int.bit_length() -> int| | Number of bits necessary to represent self in binary.| >>> bin(37)| '0b100101'| >>> (37).bit_length()| 6| | conjugate(...)| Returns self, the complex conjugate of any int.| | from_bytes(...) from builtins.type| int.from_bytes(bytes, byteorder, *, signed=False) -> int| | Return the integer represented by the given array of bytes.| | The bytes argument must be a bytes-like object (e.g. bytes or bytearray).| | The byteorder argument determines the byte order used to represent the| integer. If byteorder is 'big', the most significant byte is at the| beginning of the byte array. If byteorder is 'little', the most| significant byte is at the end of the byte array. To request the native| byte order of the host system, use `sys.byteorder' as the byte order value.| | The signed keyword-only argument indicates whether two's complement is| used to represent the integer.| | to_bytes(...)| int.to_bytes(length, byteorder, *, signed=False) -> bytes| | Return an array of bytes representing an integer.| | The integer is represented using length bytes. An OverflowError is| raised if the integer is not representable with the given number of| bytes.| | The byteorder argument determines the byte order used to represent the| integer. If byteorder is 'big', the most significant byte is at the| beginning of the byte array. If byteorder is 'little', the most| significant byte is at the end of the byte array. To request the native| byte order of the host system, use `sys.byteorder' as the byte order value.| | The signed keyword-only argument determines whether two's complement is| used to represent the integer. If signed is False and a negative integer| is given, an OverflowError is raised.| | ----------------------------------------------------------------------| Data descriptors defined here:| | denominator| the denominator of a rational number in lowest terms| | imag| the imaginary part of a complex number| | numerator| the numerator of a rational number in lowest terms| | real| the real part of a complex number

Python從內(nèi)到外評(píng)估這樣的表達(dá)式。 首先，它計(jì)算abs（-2）的值，然后它提供有關(guān)該表達(dá)式的任何值的幫助。
（事實(shí)證明有很多關(guān)于整數(shù)的說(shuō)法！在Python中，即使是簡(jiǎn)單的東西, 看起來(lái)像一個(gè)整數(shù)實(shí)際上也是一個(gè)具有相當(dāng)大內(nèi)部復(fù)雜性的對(duì)象。在稍后我們將討論P(yáng)ython中的對(duì)象，方法和屬性，上面的大量幫助輸出會(huì)更有意義。）

[4]

help(print) Help on built-in function print in module builtins:print(...)print(value, ..., sep=' ', end='\n', file=sys.stdout, flush=False)Prints the values to a stream, or to sys.stdout by default.Optional keyword arguments:file: a file-like object (stream); defaults to the current sys.stdout.sep: string inserted between values, default a space.end: string appended after the last value, default a newline.flush: whether to forcibly flush the stream.

其中一些可能看起來(lái)不可思議（什么是sys.stdout？），但是這個(gè)docstring確實(shí)揭示了我們?cè)陂_(kāi)頭的一個(gè)打印示例中使用的sep參數(shù)。

Defining functions

內(nèi)置函數(shù)非常棒，但在我們需要定義自己的函數(shù)之前，我們只能使用它們。 下面是一個(gè)簡(jiǎn)單的定義函數(shù)的例子。

[5]

def least_difference(a, b, c):diff1 = abs(a - b)diff2 = abs(b - c)diff3 = abs(a - c)return min(diff1, diff2, diff3)

這里創(chuàng)建了一個(gè)名為least_difference的函數(shù)，有三個(gè)參數(shù)（a, b, c）.

函數(shù)以def關(guān)鍵字開(kāi)頭。 調(diào)用函數(shù)時(shí)會(huì)運(yùn)行冒號(hào)后面: 的縮進(jìn)代碼塊。
return是與函數(shù)唯一關(guān)聯(lián)的另一個(gè)關(guān)鍵字。 當(dāng)Python遇到return語(yǔ)句時(shí)，它會(huì)立即退出函數(shù)，并將右側(cè)的值傳遞給調(diào)用上下文。
是否清楚了源碼中的least_difference（）做了什么？ 如果我們不確定，我們總是可以嘗試一些例子：

[6]

print(least_difference(1, 10, 100),least_difference(1, 10, 100),least_difference(5, 6, 7),#Python允許在參數(shù)列表中使用尾隨逗號(hào)，很有趣吧？ ) 9 0 1

或許help()函數(shù)可以告訴我們一些事情。

[7]

hep(least_difference) Help on function least_difference in module __main__:least_difference(a, b, c)

不出所料，Python不夠聰明，無(wú)法讀取我的代碼并將其變成一個(gè)很好的英文描述。 但是，當(dāng)我編寫一個(gè)函數(shù)時(shí)，我可以提供一個(gè)名為docstring的描述。

Docsting(文檔字符串)

[8]

def least_difference(a, b, c):"""Return the smallest difference between any two numbersamong a, b and c.>>> least_difference(1, 5, -5)4"""diff1 = abs(a - b)diff2 = abs(b - c)diff3 = abs(a - c)return min(diff1, diff2, diff3)

docstring是一個(gè)三引號(hào)""""""字符串（可能跨越多行），緊跟在函數(shù)頭之后。 當(dāng)我們?cè)诤瘮?shù)上調(diào)用help（）時(shí)，它會(huì)顯示docstring。

[9]

help(least_difference) Help on function least_difference in module __main__:least_difference(a, b, c)Return the smallest difference between any two numbersamong a, b and c.>>> least_difference(1, 5, -5)4

旁白：示例調(diào)用?docstring的最后兩行是示例函數(shù)調(diào)用和結(jié)果。 （>>>是對(duì)Python交互式shell中使用的命令提示符的引用。）Python不運(yùn)行示例調(diào)用 - 它只是為了讀者的利益。 在函數(shù)的文檔字符串中包含一個(gè)或多個(gè)示例調(diào)用的約定遠(yuǎn)非普遍觀察到，但它可以非常有效地幫助某人理解您的函數(shù)。 有關(guān)實(shí)際示例，請(qǐng)參閱numpy函數(shù)的docstring。

Docstrings是一種很好的方式來(lái)他人介紹你的代碼，甚至是你自己。 （你有多少次回到你前一天工作的一些代碼，并想知道“我在想什么？”）

Functions that don't return

如果在函數(shù)中不包含return關(guān)鍵字會(huì)怎么樣？

[10]

def least_difference(a, b, c):"""Return the smallest difference between any two numbersamong a, b and c."""diff1 = abs(a - b)diff2 = abs(b - c)diff3 = abs(a - c)min(diff1, diff2, diff3)print(least_difference(1, 10, 100),least_difference(1, 10, 10),least_difference(5, 6, 7), ) None None None

Python允許我們定義這樣的函數(shù)。 調(diào)用它們的結(jié)果是特殊值None。 （這與其他語(yǔ)言中的“null”概念類似。）
沒(méi)有return語(yǔ)句，least_difference是完全沒(méi)有意義的，但是帶有副作用的函數(shù)可以在不返回任何內(nèi)容的情況下做一些有用的事情。 我們已經(jīng)看到了兩個(gè)這樣的例子：print（）和help（）不返回任何內(nèi)容。 我們調(diào)用它們只是為了副作用（在屏幕上放置一些文字）。 其他有用的副作用示例包括寫入文件或修改輸入。

[11]

mystery = print() print(mystery) None

Default arguments

當(dāng)我們調(diào)用help(print)時(shí)，我們看到print函數(shù)有一些可選參數(shù)，例如，我們可以為sep指定一個(gè)值，在我們打印的參數(shù)之間添加一些特殊字符串：

[12]

print(1, 2, 3, sep=' < ')1 < 2 < 3

但是如果我們不指定sep的值，sep默認(rèn)值是空格' '。

[13]

print(1, 2, 3)1 2 3

將可選參數(shù)的默認(rèn)值添加到我們定義的函數(shù)中非常簡(jiǎn)單：

[14]

def greet(who="Colin"):print("Hello,", who)greet() greet(who="Kaggle") # (In this case, we don't need to specify the name of the argument, because it's unambiguous.) greet("world") Hello, Colin Hello, Kaggle Hello, world

Functions are objects too

[15]

def f(n):return n * 2x = 12.5

創(chuàng)建它們的語(yǔ)法可能不同，但上面代碼中的f和x并沒(méi)有根本不同。 它們是每個(gè)對(duì)象的引用變量。 x指的是float類型的對(duì)象，f指的是......’類型的對(duì)象好吧，讓我們問(wèn)Python：

[16]

print(type(x),type(f), sep='\n' ) <class 'float'> <class 'function'>

我們甚至可以讓Python打印出f:

[17]

print(x) print(f)12.5 <function f at 0x7fbdb18040d0>

......雖然它顯示的并不是非常有用。
請(qǐng)注意，上面的代碼單元有將另一個(gè)函數(shù)作為輸入的函數(shù)（type and print）的示例。 這開(kāi)辟了一些有趣的可能性 - 我們可以將我們收到的函數(shù)作為參數(shù)調(diào)用。

[18]

def call(fn, arg):"""Call fn on arg"""return fn(arg)def squared_call(fn, arg):"""Call fn on the result of calling fn on arg"""return fn(fn(arg))print(call(f, 1),squared_call(f, 1), sep='\n', # '\n' is the newline character - it starts a new line ) 2 4

您可能不會(huì)經(jīng)常自己定義更高階的函數(shù)，但是有一些現(xiàn)有的函數(shù)（內(nèi)置于Python和像pandas或numpy這樣的庫(kù)中），您可能會(huì)發(fā)現(xiàn)這些函數(shù)很有用。 例如，max函數(shù)。
默認(rèn)情況下，max返回其最大的參數(shù)。 但是如果我們使用可選的key參數(shù)傳入一個(gè)函數(shù)，它會(huì)返回使key（x）最大的參數(shù)x。

[19]

def mod_5(x):"""Return the remainder of x after dividing by 5"""return x % 5print('Which number is biggest?',max(100, 51, 14),'Which number is the biggest modulo 5?',max(100, 51, 14, key=mod_5),sep='\n', ) which number is biggest? 100 Which number is the biggest modulo 5? 14

Lambda functions

如果你正在編寫一個(gè)簡(jiǎn)短的函數(shù)，它的主體是單行（如上面的mod_5），Python的lambda語(yǔ)法很方便。

[20]

mod_5 = lambda x: x % 5# Note that we don't use the "return" keyword above (it's implicit) # (The line below would produce a SyntaxError) #mod_5 = lambda x: return x % 5print('101 mod 5 =', mod_5(101)) 101 mod 5 = 1

[21]

# Lambdas can take multiple comma-separated arguments abs_diff = lambda a, b: abs(a-b) print("Absolute difference of 5 and 7 is", abs_diff(5, 7)) Absolute difference of 5 and 7 is 2

[23]

# Or no arguments always_32 = lambda: 32 always_32()32

通過(guò)明智地使用lambdas，您可以偶爾在一行中解決復(fù)雜問(wèn)題。

[23]

# Preview of lists and strings. (We'll go in depth into both soon) # - len: return the length of a sequence (such as a string or list) # - sorted: return a sorted version of the given sequence (optional key # function works similarly to max and min) # - s.lower() : return a lowercase version of string s names = ['jacques', 'Ty', 'Mia', 'pui-wa'] print("Longest name is:", max(names, key=lambda name: len(name))) # or just key=len print("Names sorted case insensitive:", sorted(names, key=lambda name: name.lower())) Longest name is: jacques Names sorted case insensitive: ['jacques', 'Mia', 'pui-wa', 'Ty']

Your turn!

轉(zhuǎn)到練習(xí)筆記本，以獲得一些使用函數(shù)和獲得幫助實(shí)踐練習(xí)。

?

?

?

?

?

?

?

?

?

總結(jié)

以上是生活随笔為你收集整理的2.Functions and Getting Help的全部?jī)?nèi)容，希望文章能夠幫你解決所遇到的問(wèn)題。

如果覺(jué)得生活随笔網(wǎng)站內(nèi)容還不錯(cuò)，歡迎將生活随笔推薦給好友。