Thanks for this. But I'm not sure if this is WRONG. Chinese regular tone changes are not written according to https://resources.allsetlearning.com/chinese/pronunciation/Tone_change_rules#Why_Tone_Changes_Are_Not_Written. Instead, I think it's better to distinguish those two: original vs. rule-applied.

cedict.txt 一 only have yi1, 不 only have bu4, if you want to distinguish them, you need data

I think some simple rules can help. I'm working on them. I'll be back in hours.

@begeekmyfriend I've added the pronunciation that tone change rules are applied to. Upgrade the library to check it and please let me know if it is okay. Thanks for pointing this out.

>>> g2p("一心一意")
[[('一心一意', 
'i', 
'yi1 xin1 yi1 yi4', # this is the original pronunciation
'yi4 xin1 yi2 yi4',   # this is the descriptive pronunciation
"/concentrating one's thoughts and efforts/single-minded/bent on/intently/", 
'一心一意')]

一 and 33 to 23 actually need to predict.

for example:
有一次 -> you2 yi2 ci4, but 第一次 -> di4 yi1 ci4; the pronunciation of 一 depends on semantic

有一次 should be close to the segmentation of 一次 (not 有一 because it is not a word in Chinese) and 第一次 should be close to the segmentation of 第一.

more example:

十一{yi1}二岁来到戏校
同年十一{yi1}月
一{yi1}九八二年英文版
欧洲统一{yi1}步伐
吉林省一{yi1}号工程
一{yi1}是选拔优秀干部

@Jackiexiao Can you clarify what you mean? It's confusing. The current results for the strings above are like:

有一次
original: you3 yi1 ci4
descriptive (tone changed): you3 yi2 ci4

第一次。
original: di4 yi1 ci4 。
descriptive (tone changed): di4 yi2 ci4 。

十一二岁来到戏校
original: shi2 yi1 er4 sui4 lai2 dao4 xi4 xiao4
descriptive (tone changed): shi2 yi2 er4 sui4 lai2 dao4 xi4 xiao4

同年十一月
original: tong2 nian2 shi2 yi1 yue4
descriptive (tone changed): tong2 nian2 shi2 yi2 yue4

一九八二年英文版
original: yi1 jiu3 ba1 er4 nian2 ying1 wen2 ban3
descriptive (tone changed): yi4 jiu3 ba1 er4 nian2 ying1 wen2 ban3

欧洲统一步伐
original: ou1 zhou1 tong3 yi1 bu4 fa2
descriptive (tone changed): ou1 zhou1 tong3 yi2 bu4 fa2

吉林省一号工程
original: ji2 lin2 sheng3 yi1 hao4 gong1 cheng2
descriptive (tone changed): ji2 lin2 sheng3 yi2 hao4 gong1 cheng2

一是选拔优秀干部
original: yi1 shi4 xuan3 ba2 you1 xiu4 gan4 bu4
descriptive (tone changed): yi2 shi4 xuan3 ba2 you1 xiu4 gan4 bu4

Which parts are incorrect?

Well it is really confusing when you first learn Chinese on 一, 不 and double 3rd tone. Let me show you the rough rule on 一 base on sentences above.

有一次 means one time which is a regular word in Chinese. Therefore the tone of 一 depends on the following character 次. And 有一 is not a segmented word. So we read it as yi2 ci4.

第一次 means the first time where 第一 is a segmented word in Chinese. So we ignore 次 behind 一 and read it as di4 yi1 ci4.

十一二岁 we find that here 十一 can be segmented as a word. So we ignore the following 二 and read it as shi2 yi1 er2.

同年十一月 here 十一 can be segmented as a word so we read it as shi2 yi1 yue4.

一九八二年英文版 here 一 can be regarded as a single number character and parallel with 九、八、二. So we read it as yi1 jiu3 ba1 er4.

欧洲统一步伐 where 统一 is seperate from 欧洲 and 步伐 in Chinese words and there is no following character behind it so we read it as tong3 yi1.

吉林省一号工程 where 一号 is seperate from 吉林省 and 工程, and 一号 is not a regular word like one day or one time. It only means number one so we read it as yi1 hao4.

一是选拔优秀干部 where 一 is a single number word and 一是 is not a segmented word. So we read it as yi1.

According to https://en.wikipedia.org/wiki/Standard_Chinese_phonology#Tone_sandhi

For 一 yī:

   1.  一 is pronounced with second tone when followed by a fourth tone syllable.

        Example: 一定 (yī+dìng, "must") becomes yídìng [i˧˥tiŋ˥˩]

   2. Before a first, second or third tone syllable, 一 is pronounced with fourth tone.

        Examples：一天 (yī+tiān, "one day") becomes yìtiān [i˥˩tʰjɛn˥], 一年 (yī+nián, "one year") becomes yìnián [i˥˩njɛn˧˥], 一起 (yī+qǐ, "together") becomes yìqǐ [i˥˩t͡ɕʰi˨˩˦].

    3. When final, or when it comes at the end of a multi-syllable word (regardless of the first tone of the next word), 一 is pronounced with first tone. It also has first tone when used as an ordinal number (or part of one), and when it is immediately followed by any digit (including another 一; hence both syllables of the word 一一 yīyī and its compounds have first tone).
    4. When 一 is used between two reduplicated words, it may become neutral in tone (e.g. 看一看 kànyikàn ("to take a look of")).

So are the rules 1 and 2 applied word-internally only? In other words, when 一 is followed by a fourth-tone character which belongs to a separate word, 一 is read as first tone, not second tone?

That is right for what you have learned.

give another interesting example:

一{yi1}线城市
一{yi2}线希望

一线希望 can be regarded as a regular word in such case while 一线城市 should be segmented as 一, 线 and 城市. That is why Chinese always drives you mad.

I'm looking at the literature about the tone change rules. Unfortunately, most of them are not clear about the boundaries. But some say the tone change rules MAY work across word boundaries. If my understanding is correct, things are more complicated. If we just think all the tone change rules including third tone, 一， and 不 occur word-internally, things are simple, but I'm not sure if that's true.

I do not think one can do Chinese Pinyin conversion totally correct. There are no rules but conventions. A enoumous pinyin dictionary is indisensable in such issue. That is what we can do about it in all.

Okay. I've updated it to 0.9.9.3. I tried to refine the rules. Feel free to check it.

Hi Kyubyong,
The tone change for "一" also depends on context.
Some more complicated examples:
一(yi1)层 means the first floor; 一(yi4)层，means one floor or one layer.
一(yi1)级 means the first level (class); 一(yi4)级，means one (more or less) level

Do you consider to use machine learning like CRF to predict the tone change of 一？

Thanks.

I have found a well designed Chinese pinyin dictionary from espeak with 21567 single characters plus 36098 compound exceptions (includes 332 added 'yi' and 10720 added 'bu' exceptions, and 9713 extra 2-syllable words for 3rd-tone sandhi blocking). Would you like to replace the original one with it @Kyubyong ?