Tuesday, July 9, 2013

Unicode Tips in Python 2 and R

Most of time, I don’t need to deal with different encodings at all. When possible, I use ASCII characters. And when there is a little processing in Chinese characters or other Unicode characters, I use .Net languages or JVM languages, in which every string is Unicode and of course when the characters are displayed they are displayed as characters (not as the unreadable escaped strings or Unicode IDs).

However recently I am working on projects on Chinese Weibo data, and I encountered some Unicode problems when using Python and R. (I use Python for data crawling and processing and R for modeling and visualization.)

This post is a note I start today and I will update it when I encounter new Unicode problems…

Python 2.7

Lesson 1: Normal string and Unicode string are two types. Do not mix use them.

When writing Unicode strings literals, we put a prefix u before the string:

>>> type(u'')
<type 'unicode'>
>>> type('')
<type 'str'>

Notice that the types of the two strings are different, one is 'unicode’, and the other is ‘str’. Python is a dynamic language and sometimes will do smart things between the two types. But we should use Python as if it is a strict static type language and never cross use the two types, and when needed, do conversions explicitly. See the following example: 
>>> u'abcd'.find('b')
1
>>> u'你好'.find('好')

Traceback (most recent call last):
File "", line 1, in
u'你好'.find('好')
UnicodeDecodeError: 'ascii' codec can't decode byte 0xba in position 0: ordinal not in range(128)


The first find operation works fine because it does not involve any non-ascii characters. The second won’t work because the two types don’t work with each other.



The error in the example seems simple to avoid. However when you load a set of text files from different sources, you may mix Unicode strings and normal strings.



Lesson 2: When passing a Unicode string to a function not written by you, check carefully whether that function supports Unicode.



It seems that if we have all the strings as Unicode strings in Python 2.7, we won’t have any problems. We can add u prefix to all non-ascii string in the file, and we also use codecs package to read Unicode files:



    with codecs.open(unicode_text_file, 'r', encoding='utf-8') as f:
read the file and all strings you get are of unicode type


Everything seems fine until we call function/packages that are written only for normal strings and we wished they would work for Unicode strings. For example, the csv package in Python 2.7 official release does not work with Unicode files. You cannot do things like:



    with codecs.open(file_profile, 'r', encoding='utf-8') as f:
reader = csv.reader(f)
head = reader.next()


The third line will throw UnicodeDecodeError just as above. So in Python 2.7 this is the pain point – you cannot expect all the common libraries that work nicely with normal strings to work nicely with Unicode strings. And a script that worked before for ascii files can suddenly fail on a Unicode file.



For library writers, it is pain too. Sometimes they need to write a special version for Unicode. I once worked with a human name parser in Python. European names can have accents on letters, but that library only accepts ascii strings. To use that name parser, I have to



1. Convert the name string into a normal Python string by removing all accents



2. Parse it using the library



3. Convert the result to Unicode string using .decode method



R



Lesson 1: In R, there is only one type of string, that is character. But this type can store whatever can be repressed as byte arrays. Think of R’s character type as C’s char[] and nothing more.



In Python, we have two types for two kinds of strings. In R, we have only one type for all kinds of strings.



My first problem is displaying Unicode characters in R. R’s default GUI and command line console cannot even display Chinese on a non-Chinese Windows. (On a Chinese Windows, you can only get a Chinese version R. Chinese characters are fine. However all info/error messages are also in Chinese, which are translated from English and which are just wired Chinese.)



When R is installed, it checks the System encoding. On Linux and Mac OS, the system encoding is usually UTF-8, and R uses that. However Windows is different, the following is my R session info:



> sessionInfo()
R version 2.15.1 (2012-06-22)
Platform: x86_64-pc-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=English_United States.1252 LC_CTYPE=English_United States.1252
[3] LC_MONETARY=English_United States.1252 LC_NUMERIC=C
[5] LC_TIME=English_United States.1252


R’s default GUI then uses the encoding to display all strings. Of course, it fails on most Unicode characters.



Luckily the editor and the console in RStudio can display Chinese characters. And RStudio must have done some smart things:



> as.character(china_mapdata$NAME[1])
[1] "黑龙江省"
> "黑龙江省"
[1] "黑龙江省"
> as.character(china_mapdata$NAME[1]) == "黑龙江省"
[1] FALSE


The string in the data frame displays the same as the literal (the literal is represented as Unicode in RStudio console), yet they are not equal in many ways. This is because two strings display the same can be different in their internal representations:



> nchar("黑龙江省")
[1] 4
> nchar(as.character(china_mapdata$NAME[1]))
[1] 8


Their types are both character. However the representation are different: the literal is represented as 4 Unicode characters (4 chars * 3 bytes/char = 12 bytes) in the memory, and the string read from the data file is represented as 8 bytes (4 chars * 2 bytes/char = 8 bytes) in the memory:



> charToRaw("黑龙江省")
[1] e9 bb 91 e9 be 99 e6 b1 9f e7 9c 81
> charToRaw(as.character(china_mapdata$NAME[1]))
[1] ba da c1 fa bd ad ca a1


I find that the Chinese characters in the file I load the data frame uses GB2312 encoding. And because it is a binary file, I don’t have a simple way to change its encoding. But here is a method that I find:



# First write the data frame to disk
write.csv(china_mapdata, file = 'china_map.csv')
# In EmEditor, open it as GB2312, and SaveAs UTF-8
# Load the utf-8 file
china_mapdata <- read.csv('china_map.utf8.csv', encoding='UTF-8')
# Test
as.character(china_mapdata$NAME[1]) == "黑龙江省" # should be TRUE now


After converting all Chinese characters in Unicode, I can now follow the map example here (the author of the post, I believe, uses a Chinese Windows, therefore does not have my problem; All systems except Chinese Windows will encounter the coding problem though) :



ggplot(zhejiang, aes(x = long, y = lat, group = group,fill=NAME))+
geom_polygon(fill="beige" )+
geom_path(colour = "grey40")+
ggtitle("中华人民共和国浙江省")+
geom_point(x=120.12,y=30.16,fill=FALSE)+
annotate("text",x=120.3,y=30,label="杭州市")


But when exporting the file as PDF, the Chinese characters cannot display correctly: 



image



After searching online, I find the solution in this SO question, and specifying the font explicitly solves the problem:



cairo_pdf("example.pdf", family="FangSong")
ggplot(zhejiang, aes(x = long, y = lat, group = group,fill=NAME))+
geom_polygon(fill="beige" )+
geom_path(colour = "grey40")+
ggtitle("中华人民共和国浙江省")+
geom_point(x=120.12,y=30.16,fill=FALSE)+
annotate("text",x=120.3,y=30,label="杭州市")
dev.off()


The correct PDF output:



image



We can also change the font type to other Chinese fonts, refer their names here.



Some R’s functions can recognize the Unicode string, e.g. Encoding. I think such recognition is based on the first few bits of the string. But it does not recognize GB2312 string (Encoding function outputs ‘unknown’ for GB2312 strings). Magically RStudio in Windows (English version, locale set to Simplified Chinese) can recognize both strings and display them correctly.



Lesson 2: When dealing with Unicode in R, use a Chinese Windows (for Chinese only), or use Linux/Mac OS (which is by default UTF-8), otherwise you cannot display Unicode well.



See how Unicode code may be displayed in R's console (English Windows):



> head(zhejiang)
X.U.FEFF. AREA PERIMETER BOU2_4M_ BOU2_4M_ID ADCODE93 ADCODE99 NAME
223 222 9.277 26.825 224 33 330000 330000 <u +6D59><u +6C5F><u +7701>
224 223 0.000 0.103 225 1626 330000 330000 <u +6D59><u +6C5F><u +7701>
225 224 0.000 0.052 226 1634 330000 330000 <u +6D59><u +6C5F><u +7701>


The <u+xxx> symbols indicate that R knows that they are Unicode characters, however R cannot display them correctly. And these symbols occur as symbols in your plots as well, which is just meaningless. If R cannot display these characters as characters, how can we know what are they and how to use these characters in a plot?



To solve this problem, my solution is to use a Linux with UTF-8 encoding. This is the session info in my Linux virtual machine:



> sessionInfo()
R version 2.14.1 (2011-12-22)
Platform: i686-pc-linux-gnu (32-bit)

locale:
[1] LC_CTYPE=en_US.UTF-8 LC_NUMERIC=C LC_TIME=en_US.UTF-8 LC_COLLATE=en_US.UTF-8
[5] LC_MONETARY=en_US.UTF-8 LC_MESSAGES=en_US.UTF-8 LC_PAPER=C LC_NAME=C
[9] LC_ADDRESS=C LC_TELEPHONE=C LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C


Summary



Conceptually Unicode in Python and R are quite simple. In Python, Unicode and normal strings have different types. In R, all the strings are represented as byte arrays (the good old C char[]!); so from its type, you cannot decide whether a string is Unicode or not. I think Python’s two-type design is good because the runtime exception will force the programmer to split normal strings and Unicode strings clearly (though it is headache, this is why Python 3 treat all strings as Unicode strings) .



What adds to the Unicode complexity is functions that manipulate Unicode strings and especially the display of Unicode characters on screens. A Unicode character is anyway 2-4 bytes in the memory, nothing special. When using Python and R for data analysis, we usually follow the REPL (Read–eval–print loop), it is meaningless to display Unicode as escaped strings. One cannot even judge whether the character is a Chinese character by reading the escaped strings! What’s worse is that different terminals may display differently for the same character. For example, in a Unicode-enabled Python console:



>>> print u'你'

>>> u'你'
u'\u4f60'


In a non-Unicode console:



>>> u'你'
u'\xc4\xe3'
>>> print u'你'
Äã
>>> print u'\u4f60'

>>> u'你' == u'\u4f60'
False
(In this example, the problem is not that the terminal cannot display Unicode characters, it is that the input characters in the terminal are not Unicode!)


The suggestion for the terminal and editor is



Use a Unicode terminal and a Unicode text editor when working with Python and R. For example, RStudio is, while Rgui.exe isn’t. PyDev plugin/PyScripter is, while the default IDLE isn’t.



And in Python, always put



# -*- coding: utf-8 -*-


at the beginning of the file, and save all source code files as Unicode.

1 comment:

  1. These are indeed helpful unicode tips. Those that are in data entry can also benefit from this information. Thanks for posting.

    ReplyDelete