csv
文件格式
源代码： Lib/csv.py
The so-called CSV (Comma Separated Values) format is the most common import and export format for spreadsheets and databases. CSV format was used for many years prior to attempts to describe the format in a standardized way in RFC 4180 . The lack of a well-defined standard means that subtle differences often exist in the data produced and consumed by different applications. These differences can make it annoying to process CSV files from multiple sources. Still, while the delimiters and quoting characters vary, the overall format is similar enough that it is possible to write a single module which can efficiently manipulate such data, hiding the details of reading and writing the data from the programmer.
The csv module implements classes to read and write tabular data in CSV format. It allows programmers to say, “write this data in the format preferred by Excel,” or “read data from this file which was generated by Excel,” without knowing the precise details of the CSV format used by Excel. Programmers can also describe the CSV formats understood by other applications or define their own special-purpose CSV formats.
The csv 模块的 reader and writer objects read and write sequences. Programmers can also read and write data in dictionary form using the DictReader and DictWriter 类。
reader
writer
DictReader
DictWriter
另请参阅
The Python Enhancement Proposal which proposed this addition to Python.
The csv 模块定义了下列函数：
返回 reader object that will process lines from the given csvfile . A csvfile must be an iterable of strings, each in the reader’s defined csv format. A csvfile is most commonly a file-like object or list. If csvfile is a file object, it should be opened with newline='' . [ 1 ] An optional dialect parameter can be given which is used to define a set of parameters specific to a particular CSV dialect. It may be an instance of a subclass of the Dialect class or one of the strings returned by the list_dialects() function. The other optional fmtparams keyword arguments can be given to override individual formatting parameters in the current dialect. For full details about the dialect and formatting parameters, see section 方言和格式化参数 .
newline=''
Dialect
list_dialects()
Each row read from the csv file is returned as a list of strings. No automatic data type conversion is performed unless the QUOTE_NONNUMERIC format option is specified (in which case unquoted fields are transformed into floats).
QUOTE_NONNUMERIC
A short usage example:
>>> import csv >>> with open('eggs.csv', newline='') as csvfile: ... spamreader = csv.reader(csvfile, delimiter=' ', quotechar='|') ... for row in spamreader: ... print(', '.join(row)) Spam, Spam, Spam, Spam, Spam, Baked Beans Spam, Lovely Spam, Wonderful Spam
Return a writer object responsible for converting the user’s data into delimited strings on the given file-like object. csvfile can be any object with a write() 方法。若 csvfile is a file object, it should be opened with newline='' [ 1 ] 。可选 dialect parameter can be given which is used to define a set of parameters specific to a particular CSV dialect. It may be an instance of a subclass of the Dialect class or one of the strings returned by the list_dialects() function. The other optional fmtparams keyword arguments can be given to override individual formatting parameters in the current dialect. For full details about dialects and formatting parameters, see the 方言和格式化参数 section. To make it as easy as possible to interface with modules which implement the DB API, the value None is written as the empty string. While this isn’t a reversible transformation, it makes it easier to dump SQL NULL data values to CSV files without preprocessing the data returned from a cursor.fetch* call. All other non-string data are stringified with str() before being written.
write()
None
cursor.fetch*
str()
import csv with open('eggs.csv', 'w', newline='') as csvfile: spamwriter = csv.writer(csvfile, delimiter=' ', quotechar='|', quoting=csv.QUOTE_MINIMAL) spamwriter.writerow(['Spam'] * 5 + ['Baked Beans']) spamwriter.writerow(['Spam', 'Lovely Spam', 'Wonderful Spam'])
Associate dialect with name . name must be a string. The dialect can be specified either by passing a sub-class of Dialect , or by fmtparams keyword arguments, or both, with keyword arguments overriding parameters of the dialect. For full details about dialects and formatting parameters, see section 方言和格式化参数 .
Delete the dialect associated with name from the dialect registry. An Error 被引发若 name is not a registered dialect name.
Error
Return the dialect associated with name . An Error 被引发若 name is not a registered dialect name. This function returns an immutable Dialect .
Return the names of all registered dialects.
Returns the current maximum field size allowed by the parser. If new_limit is given, this becomes the new limit.
The csv module defines the following classes:
Create an object that operates like a regular reader but maps the information in each row to a dict whose keys are given by the optional fieldnames 参数。
dict
The fieldnames parameter is a sequence 。若 fieldnames is omitted, the values in the first row of file f will be used as the fieldnames and will be omitted from the results. If fieldnames is provided, they will be used and the first row will be included in the results. Regardless of how the fieldnames are determined, the dictionary preserves their original ordering.
If a row has more fields than fieldnames, the remaining data is put in a list and stored with the fieldname specified by restkey (which defaults to None ). If a non-blank row has fewer fields than fieldnames, the missing values are filled-in with the value of restval (which defaults to None ).
All other optional or keyword arguments are passed to the underlying reader 实例。
If the argument passed to fieldnames is an iterator, it will be coerced to a list .
list
3.6 版改变： Returned rows are now of type OrderedDict .
OrderedDict
3.8 版改变： Returned rows are now of type dict .
>>> import csv >>> with open('names.csv', newline='') as csvfile: ... reader = csv.DictReader(csvfile) ... for row in reader: ... print(row['first_name'], row['last_name']) ... Eric Idle John Cleese >>> print(row) {'first_name': 'John', 'last_name': 'Cleese'}
Create an object which operates like a regular writer but maps dictionaries onto output rows. The fieldnames parameter is a sequence of keys that identify the order in which values in the dictionary passed to the writerow() method are written to file f 。可选 restval parameter specifies the value to be written if the dictionary is missing a key in fieldnames . If the dictionary passed to the writerow() method contains a key not found in fieldnames , the optional extrasaction parameter indicates what action to take. If it is set to 'raise' , the default value, a ValueError is raised. If it is set to 'ignore' , extra values in the dictionary are ignored. Any other optional or keyword arguments are passed to the underlying writer 实例。
sequence
writerow()
'raise'
ValueError
'ignore'
注意，不像 DictReader 类， fieldnames 参数为 DictWriter class is not optional.
import csv with open('names.csv', 'w', newline='') as csvfile: fieldnames = ['first_name', 'last_name'] writer = csv.DictWriter(csvfile, fieldnames=fieldnames) writer.writeheader() writer.writerow({'first_name': 'Baked', 'last_name': 'Beans'}) writer.writerow({'first_name': 'Lovely', 'last_name': 'Spam'}) writer.writerow({'first_name': 'Wonderful', 'last_name': 'Spam'})
The Dialect class is a container class whose attributes contain information for how to handle doublequotes, whitespace, delimiters, etc. Due to the lack of a strict CSV specification, different applications produce subtly different CSV data. Dialect instances define how reader and writer instances behave.
All available Dialect names are returned by list_dialects() , and they can be registered with specific reader and writer classes through their initializer ( __init__ ) functions like this:
__init__
import csv with open('students.csv', 'w', newline='') as csvfile: writer = csv.writer(csvfile, dialect='unix')
The excel class defines the usual properties of an Excel-generated CSV file. It is registered with the dialect name 'excel' .
excel
'excel'
The excel_tab class defines the usual properties of an Excel-generated TAB-delimited file. It is registered with the dialect name 'excel-tab' .
excel_tab
'excel-tab'
The unix_dialect class defines the usual properties of a CSV file generated on UNIX systems, i.e. using '\n' as line terminator and quoting all fields. It is registered with the dialect name 'unix' .
unix_dialect
'\n'
'unix'
Added in version 3.2.
The Sniffer class is used to deduce the format of a CSV file.
Sniffer
The Sniffer class provides two methods:
Analyze the given sample and return a Dialect subclass reflecting the parameters found. If the optional delimiters parameter is given, it is interpreted as a string containing possible valid delimiter characters.
Analyze the sample text (presumed to be in CSV format) and return True if the first row appears to be a series of column headers. Inspecting each column, one of two key criteria will be considered to estimate if the sample contains a header:
True
the second through n-th rows contain numeric values
the second through n-th rows contain strings where at least one value’s length differs from that of the putative header of that column.
Twenty rows after the first row are sampled; if more than half of columns + rows meet the criteria, True 被返回。
注意
This method is a rough heuristic and may produce both false positives and negatives.
An example for Sniffer use:
with open('example.csv', newline='') as csvfile: dialect = csv.Sniffer().sniff(csvfile.read(1024)) csvfile.seek(0) reader = csv.reader(csvfile, dialect) # ... process CSV file contents here ...
The csv module defines the following constants:
Instructs writer objects to quote all fields.
Instructs writer objects to only quote those fields which contain special characters such as delimiter , quotechar or any of the characters in lineterminator .
Instructs writer objects to quote all non-numeric fields.
Instructs reader objects to convert all non-quoted fields to type float .
Instructs writer objects to never quote fields. When the current delimiter occurs in output data it is preceded by the current escapechar character. If escapechar is not set, the writer will raise Error if any characters that require escaping are encountered.
Instructs reader objects to perform no special processing of quote characters.
Instructs writer objects to quote all fields which are not None . This is similar to QUOTE_ALL , except that if a field value is None an empty (unquoted) string is written.
QUOTE_ALL
Instructs reader objects to interpret an empty (unquoted) field as None and to otherwise behave as QUOTE_ALL .
3.12 版添加。
Instructs writer objects to always place quotes around fields which are strings. This is similar to QUOTE_NONNUMERIC , except that if a field value is None an empty (unquoted) string is written.
Instructs reader objects to interpret an empty (unquoted) string as None and to otherwise behave as QUOTE_NONNUMERIC .
The csv 模块定义以下异常：
Raised by any of the functions when an error is detected.
To make it easier to specify the format of input and output records, specific formatting parameters are grouped together into dialects. A dialect is a subclass of the Dialect class containing various attributes describing the format of the CSV file. When creating reader or writer objects, the programmer can specify a string or a subclass of the Dialect class as the dialect parameter. In addition to, or instead of, the dialect parameter, the programmer can also specify individual formatting parameters, which have the same names as the attributes defined below for the Dialect 类。
Dialects support the following attributes:
A one-character string used to separate fields. It defaults to ',' .
','
Controls how instances of quotechar appearing inside a field should themselves be quoted. When True , the character is doubled. When False ， escapechar is used as a prefix to the quotechar 。默认为 True .
False
On output, if doublequote is False and no escapechar 有设置， Error is raised if a quotechar is found in a field.
A one-character string used by the writer to escape the delimiter if quoting 被设为 QUOTE_NONE 和 quotechar if doublequote is False . On reading, the escapechar removes any special meaning from the following character. It defaults to None , which disables escaping.
QUOTE_NONE
3.11 版改变： 空 escapechar is not allowed.
The string used to terminate lines produced by the writer 。默认为 '\r\n' .
'\r\n'
The reader is hard-coded to recognise either '\r' or '\n' as end-of-line, and ignores lineterminator . This behavior may change in the future.
'\r'
A one-character string used to quote fields containing special characters, such as the delimiter or quotechar , or which contain new-line characters. It defaults to '"' .
'"'
3.11 版改变： 空 quotechar is not allowed.
Controls when quotes should be generated by the writer and recognised by the reader. It can take on any of the QUOTE_* constants and defaults to QUOTE_MINIMAL .
QUOTE_MINIMAL
当 True , spaces immediately following the delimiter are ignored. The default is False .
当 True , raise exception Error on bad CSV input. The default is False .
Reader objects ( DictReader instances and objects returned by the reader() function) have the following public methods:
reader()
Return the next row of the reader’s iterable object as a list (if the object was returned from reader() ) or a dict (if it is a DictReader instance), parsed according to the current Dialect . Usually you should call this as next(reader) .
next(reader)
Reader objects have the following public attributes:
A read-only description of the dialect in use by the parser.
The number of lines read from the source iterator. This is not the same as the number of records returned, as records can span multiple lines.
DictReader objects have the following public attribute:
If not passed as a parameter when creating the object, this attribute is initialized upon first access or when the first record is read from the file.
writer objects ( DictWriter instances and objects returned by the writer() function) have the following public methods. A row must be an iterable of strings or numbers for writer objects and a dictionary mapping fieldnames to strings or numbers (by passing them through str() first) for DictWriter objects. Note that complex numbers are written out surrounded by parens. This may cause some problems for other programs which read CSV files (assuming they support complex numbers at all).
writer()
写入 row parameter to the writer’s file object, formatted according to the current Dialect . Return the return value of the call to the write method of the underlying file object.
3.5 版改变： Added support of arbitrary iterables.
Write all elements in rows (an iterable of row objects as described above) to the writer’s file object, formatted according to the current dialect.
Writer objects have the following public attribute:
A read-only description of the dialect in use by the writer.
DictWriter objects have the following public method:
Write a row with the field names (as specified in the constructor) to the writer’s file object, formatted according to the current dialect. Return the return value of the csvwriter.writerow() call used internally.
csvwriter.writerow()
3.8 版改变： writeheader() now also returns the value returned by the csvwriter.writerow() method it uses internally.
writeheader()
The simplest example of reading a CSV file:
import csv with open('some.csv', newline='') as f: reader = csv.reader(f) for row in reader: print(row)
Reading a file with an alternate format:
import csv with open('passwd', newline='') as f: reader = csv.reader(f, delimiter=':', quoting=csv.QUOTE_NONE) for row in reader: print(row)
The corresponding simplest possible writing example is:
import csv with open('some.csv', 'w', newline='') as f: writer = csv.writer(f) writer.writerows(someiterable)
由于 open() is used to open a CSV file for reading, the file will by default be decoded into unicode using the system default encoding (see locale.getencoding() ). To decode a file using a different encoding, use the encoding argument of open:
open()
locale.getencoding()
encoding
import csv with open('some.csv', newline='', encoding='utf-8') as f: reader = csv.reader(f) for row in reader: print(row)
The same applies to writing in something other than the system default encoding: specify the encoding argument when opening the output file.
Registering a new dialect:
import csv csv.register_dialect('unixpwd', delimiter=':', quoting=csv.QUOTE_NONE) with open('passwd', newline='') as f: reader = csv.reader(f, 'unixpwd')
A slightly more advanced use of the reader — catching and reporting errors:
import csv, sys filename = 'some.csv' with open(filename, newline='') as f: reader = csv.reader(f) try: for row in reader: print(row) except csv.Error as e: sys.exit('file {}, line {}: {}'.format(filename, reader.line_num, e))
And while the module doesn’t directly support parsing strings, it can easily be done:
import csv for row in csv.reader(['one,two,three']): print(row)
脚注
configparser — 配置文件剖析器
configparser
键入搜索术语或模块、类、函数名称。