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這篇文章主要講解了Python中如何實(shí)現(xiàn)格式化字符串,內(nèi)容清晰明了,對(duì)此有興趣的小伙伴可以學(xué)習(xí)一下,相信大家閱讀完之后會(huì)有幫助。
關(guān)于Python的格式化字符串,幾乎所有接觸過(guò)Python語(yǔ)言的人都知道其中一種,即使用運(yùn)算符%,但對(duì)于絕大多數(shù)初學(xué)者來(lái)說(shuō)也僅此而已。
因此,本文將先總結(jié)如何通過(guò)%運(yùn)算符來(lái)格式化字符串,同時(shí)指出這種方式的缺點(diǎn),然后帶你了解Python中另外三種強(qiáng)大的格式化字符串的方式:str.format()、f-string以及模板字符串,并給出在何時(shí)選擇何種方式的建議。
一、%運(yùn)算符格式化字符串
1. 如何使用
字符串對(duì)象都有一個(gè)使用%運(yùn)算符完成的內(nèi)置操作,可被用來(lái)格式化字符串。最簡(jiǎn)單的如:
In [11]: name = "Monty Python" In [12]: "Hello, %s." % name Out[12]: 'Hello, Monty Python.'
如果想要對(duì)一段字符串中插入多個(gè)變量進(jìn)行格式化,則需要使用一個(gè)元組將待插入變量包在一起,如:
In [14]: name = "Monty Python" In [15]: age = 100 In [16]: "Hello, %s. You are %d years old" % (name, age) Out[16]: 'Hello, Monty Python. You are 100 years old'
2. 缺點(diǎn)概述
使用%運(yùn)算符的方式來(lái)格式化字符串自Python語(yǔ)言誕生之日起就已存在,上述代碼看起來(lái)也很直觀易讀,但是當(dāng)字符串更長(zhǎng),待插入變量更多,則使用%來(lái)格式化字符串的可讀性將急劇下降,如:
In [23]: first_name = "Eric" In [24]: last_name = "Idle" In [25]: age = 100 In [26]: profession = "comedian" In [27]: affiliation = "Monty Python" In [28]: "Hello, %s %s. You are %s. You are a %s. You were a member of %s." % (first_name, last_name, age, profession, affiliation) Out[28]: 'Hello, Eric Idle. You are 100. You are a comedian. You were a member of Monty Python.'
上述使用%格式化字符串不僅冗長(zhǎng),而且容易出錯(cuò),因?yàn)檫@種方式并不夠正確顯示元組或者字典。
實(shí)際上,在Python官方文檔中,對(duì)于使用%運(yùn)算符格式化字符串這種方式的評(píng)價(jià)也是負(fù)面的:
Each of these alternatives provides their own trade-offs and benefits of simplicity, flexibility, and/or extensibility.
當(dāng)然,上述三種可替代的格式化字符串方式也都在簡(jiǎn)潔、靈活和可擴(kuò)展方面有所取舍。
二、str.format()格式化字符串
1. 如何使用
str.format()是對(duì)使用%實(shí)現(xiàn)格式化字符串的一種改進(jìn)。這種方式使用的語(yǔ)法和普通函數(shù)調(diào)用相差無(wú)幾。
使用str.format(),字符串中待替換的域使用{}表示:
In [29]: name = "Eric" In [30]: age = 100 In [31]: "Hello, {}. You are {}.".format(name, age) Out[31]: 'Hello, Eric. You are 100.'
也可以通過(guò)索引的方式指定format()中哪一個(gè)變量和值應(yīng)該填入哪一個(gè){},如:
In [32]: name = "Eric" In [33]: age = 100 In [34]: "Hello, {1}. You are {0}.".format(age, name) Out[34]: 'Hello, Eric. You are 100.'
除了索引,也可以通過(guò)在{}中指定名稱的方式來(lái)實(shí)現(xiàn)類似上述功能,如:
In [36]: name = "Eric" In [37]: age = 100 In [38]: "Hello, {name}. You are {age}.".format(age=age, name=name) Out[38]: 'Hello, Eric. You are 100.'
基于上面的方式,當(dāng)待格式化的信息都來(lái)自一個(gè)字典時(shí),Python中還有如下騷操作:
In [39]: person = {'name': 'Eric', 'age': 100} In [40]: "Hello, {name}. You are {age}.".format(name=person['name'], age=person['age']) Out[40]: 'Hello, Eric. You are 100.'
更騷的是,上面的騷操作還能利用字典拆包**進(jìn)一步簡(jiǎn)化:
In [41]: person = {'name': 'Eric', 'age': 100} In [42]: "Hello, {name}. You are {age}.".format(**person) Out[42]: 'Hello, Eric. You are 100.'
2. 缺點(diǎn)概述
相較于%運(yùn)算符方式,使用str.format()已經(jīng)使得格式化字符串語(yǔ)法更加可讀,但是當(dāng)變量增多時(shí),這種方式寫(xiě)出的程序也會(huì)顯得很冗長(zhǎng):
In [43]: first_name = "Eric" In [44]: last_name = "Idle" In [45]: age = 74 In [46]: profession = "comedian" In [47]: affiliation = "Monty Python" In [48]: print(("Hello, {first_name} {last_name}. You are {age}. " + ...: "You are a {profession}. You were a member of {affiliation}.") \ ...: .format(first_name=first_name, last_name=last_name, age=age, \ ...: profession=profession, affiliation=affiliation)) Hello, Eric Idle. You are 74. You are a comedian. You were a member of Monty Python.
三、f-string格式化字符串
在Python 3.6中f-string被引入(具體請(qǐng)見(jiàn)PEP 498),也被稱作格式化字符串字面量(formatted string literals)。
f-string是字符串字面量,且其以字母f開(kāi)頭,{}中包含變量或表達(dá)式,變量或表達(dá)式將在運(yùn)行(runtime)時(shí)通過(guò)使用__format__協(xié)議被替換成具體的值。
1. 如何使用
簡(jiǎn)單的f-string格式化字符串如:
In [49]: name = "Eric" In [50]: age = 100 In [51]: f"Hello, {name}. You are {age}." Out[51]: 'Hello, Eric. You are 100.'
如前所述,{}中除接受變量外,還接受表達(dá)式,如:
In [52]: f"{2 * 37}" Out[52]: '74'
f-string更強(qiáng)大的地方在于,{}中接受函數(shù)調(diào)用:
In [53]: def to_lowercase(input): ...: return input.lower() ...: In [54]: name = "Eric Idle" In [55]: f"{to_lowercase(name)} is funny." Out[55]: 'eric idle is funny.'
甚至,你可以對(duì)創(chuàng)建于類的對(duì)象使用f-string,如:
class Comedian: def __init__(self, first_name, last_name, age): self.first_name = first_name self.last_name = last_name self.age = age def __str__(self): return f"{self.first_name} {self.last_name} is {self.age}." def __repr__(self): return f"{self.first_name} {self.last_name} is {self.age}. Surprise!" def main(): new_comedian = Comedian("Eric", "Idle", "74") print(f"{new_comedian}") if __name__ == '__main__': main()
上述代碼的輸出為:
Eric Idle is 74.
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