13.1. csv — Leiture e Escrita de arquivos CSV

Novo na versão 2.3.

The so-called CSV (Comma Separated Values) format is the most common import and export format for spreadsheets and databases. There is no “CSV standard”, so the format is operationally defined by the many applications which read and write it. The lack of a 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.

O módulo csv implementa classes para ler e gravar dados tabulares no formato CSV. Ele permite que os programadores digam “escreva esses dados no formato preferido pelo Excel” ou “leia os dados desse arquivo gerado pelo Excel”, sem conhecer os detalhes precisos do formato CSV usado pelo Excel. Os programadores também podem descrever os formatos CSV entendidos por outros aplicativos ou definir seus próprios formatos CSV para fins especiais.

Os objetos de reader e writer do módulo csv leem e escrevem sequências. Os programadores também podem ler e gravar dados no formato de dicionário usando as classes DictReader e DictWriter.

Nota

This version of the csv module doesn’t support Unicode input. Also, there are currently some issues regarding ASCII NUL characters. Accordingly, all input should be UTF-8 or printable ASCII to be safe; see the examples in section Exemplos.

Ver também

PEP 305 - API de arquivo CSV

A proposta de melhoria do Python que propôs essa adição ao Python.

13.1.1. Conteúdo do Módulo

O módulo csv define as seguintes funções:

csv.reader(csvfile, dialect='excel', **fmtparams)

Return a reader object which will iterate over lines in the given csvfile. csvfile can be any object which supports the iterator protocol and returns a string each time its next() method is called — file objects and list objects are both suitable. If csvfile is a file object, it must be opened with the ‘b’ flag on platforms where that makes a difference. 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 Dialetos e parâmetros de formatação.

Each row read from the csv file is returned as a list of strings. No automatic data type conversion is performed.

Um pequeno exemplo de utilização

>>> import csv
>>> with open('eggs.csv', 'rb') 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

Alterado na versão 2.5: The parser is now stricter with respect to multi-line quoted fields. Previously, if a line ended within a quoted field without a terminating newline character, a newline would be inserted into the returned field. This behavior caused problems when reading files which contained carriage return characters within fields. The behavior was changed to return the field without inserting newlines. As a consequence, if newlines embedded within fields are important, the input should be split into lines in a manner which preserves the newline characters.

csv.writer(csvfile, dialect='excel', **fmtparams)

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() method. If csvfile is a file object, it must be opened with the ‘b’ flag on platforms where that makes a difference. 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 Dialetos e parâmetros de formatação. 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. Floats are stringified with repr() before being written. All other non-string data are stringified with str() before being written.

Um pequeno exemplo de utilização

import csv
with open('eggs.csv', 'wb') 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'])

csv.register_dialect(name, [dialect, ]**fmtparams)

Associate dialect with name. name must be a string or Unicode object. 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 the dialect and formatting parameters, see section Dialetos e parâmetros de formatação.

csv.unregister_dialect(name)

Exclui o dialeto associado ao name do registro do dialeto. Um Error é levantado se name não for um nome de dialeto registrado.

csv.get_dialect(name)

Return the dialect associated with name. An Error is raised if name is not a registered dialect name.

Alterado na versão 2.5: This function now returns an immutable Dialect. Previously an instance of the requested dialect was returned. Users could modify the underlying class, changing the behavior of active readers and writers.

csv.list_dialects()

Retorna os nomes de todos os dialetos registrados

csv.field_size_limit([new_limit])

Retorna o tamanho máximo atual do campo permitido pelo analisador sintático. Se new_limit for fornecido, este se tornará o novo limite.

Novo na versão 2.5.

O módulo csv define as seguintes classes:

class csv.DictReader(f, fieldnames=None, restkey=None, restval=None, dialect='excel', *args, **kwds)

Create an object which operates like a regular reader but maps the information read into a dict whose keys are given by the optional fieldnames parameter. The fieldnames parameter is a sequence whose elements are associated with the fields of the input data in order. These elements become the keys of the resulting dictionary. If the fieldnames parameter is omitted, the values in the first row of the file f will be used as the fieldnames. If the row read has more fields than the fieldnames sequence, the remaining data is added as a sequence keyed by the value of restkey. If the row read has fewer fields than the fieldnames sequence, the remaining keys take the value of the optional restval parameter. Any other optional or keyword arguments are passed to the underlying reader instance.

Um pequeno exemplo de utilização

>>> import csv
>>> with open('names.csv') as csvfile:
...     reader = csv.DictReader(csvfile)
...     for row in reader:
...         print(row['first_name'], row['last_name'])
...
Baked Beans
Lovely Spam
Wonderful Spam

class csv.DictWriter(f, fieldnames, restval='', extrasaction='raise', dialect='excel', *args, **kwds)

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 the file f. The optional 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' 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 instance.

Note that unlike the DictReader class, the fieldnames parameter of the DictWriter is not optional. Since Python’s dict objects are not ordered, there is not enough information available to deduce the order in which the row should be written to the file f.

Um pequeno exemplo de utilização

import csv

with open('names.csv', 'w') 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'})

class csv.Dialect

A classe Dialect é uma classe de contêiner baseada principalmente em seus atributos, que são usados para definir os parâmetros para uma instância específica reader ou writer.

class csv.excel

A classe excel define as propriedades usuais de um arquivo CSV gerado pelo Excel. Ele é registrado com o nome do dialeto 'excel'.

class csv.excel_tab

A classe excel_tab define as propriedades usuais de um arquivo delimitado por TAB gerado pelo Excel. Ela é registrado com o nome do dialeto 'excel-tab'.

class csv.Sniffer

A classe Sniffer é usada para deduzir o formato de um arquivo CSV.

A classe Sniffer fornece dois métodos:

sniff(sample, delimiters=None)

Analisa a sample fornecida e retorna uma subclasse Dialect, refletindo os parâmetros encontrados. Se o parâmetro opcional delimiters for fornecido, ele será interpretado como uma string contendo possíveis caracteres válidos de delimitador.

has_header(sample)

Analisa o texto de amostra (presumivelmente no formato CSV) e retorna True se a primeira linha parecer ser uma série de cabeçalhos de coluna.

Um exemplo para uso de Sniffer:

with open('example.csv', 'rb') as csvfile:
    dialect = csv.Sniffer().sniff(csvfile.read(1024))
    csvfile.seek(0)
    reader = csv.reader(csvfile, dialect)
    # ... process CSV file contents here ...

O módulo csv define as seguintes constantes:

csv.QUOTE_ALL

Instrui objetos writer a colocar aspas em todos os campos.

csv.QUOTE_MINIMAL

Instrui objetos writer a colocar aspas apenas nos campos que contêm caracteres especiais como delimiters, quotechar ou qualquer um dos caracteres em lineterminator.

csv.QUOTE_NONNUMERIC

Instrui objetos writer a colocar aspas em todos os campos não numéricos.

Instrui o leitor a converter todos os campos não citados no tipo float.

csv.QUOTE_NONE

Instrui objetos writer a nunca colocar aspas nos campos. Quando o delimiter atual ocorre nos dados de saída, é precedido pelo caractere escapechar atual. Se escapeechar não estiver definido, o escritor levantará Error se algum caractere que exija escape for encontrado.

Instrui reader a não executar nenhum processamento especial de caracteres de aspas.

O módulo csv define a seguinte exceção:

exception csv.Error

Levantada por qualquer uma das funções quando um erro é detectado.

13.1.2. Dialetos e parâmetros de formatação

Para facilitar a especificação do formato dos registros de entrada e saída, parâmetros específicos de formatação são agrupados em dialetos. Um dialeto é uma subclasse da classe Dialect com um conjunto de métodos específicos e um único método validate(). Ao criar objetos reader ou writer, o programador pode especificar uma string ou uma subclasse da classe Dialect como parâmetro de dialeto. Além do parâmetro dialect, ou em vez do parâmetro dialect, o programador também pode especificar parâmetros de formatação individuais, com os mesmos nomes dos atributos definidos abaixo para a classe Dialect.

Os dialetos possuem suporte aos seguintes atributos:

Dialect.delimiter

Uma string de um caractere usada para separar campos. O padrão é ','.

Dialect.doublequote

Controla como as instâncias de quotechar que aparecem dentro de um campo devem estar entre aspas. Quando True, o caractere é dobrado. Quando False, o escapechar é usado como um prefixo para o quotechar. O padrão é True.

Na saída, se doublequote for False e não escapeechar for definido, Error é levantada se um quotechar é encontrado em um campo.

Dialect.escapechar

Uma string usada pelo escritor para escapar do delimitator se quoting estiver definida como QUOTE_NONE e o quotechar se quoting for False. Na leitura, o escapechar remove qualquer significado especial do caractere seguinte. O padrão é None, que desativa o escape.

Dialect.lineterminator

A string usada para terminar as linhas produzidas pelo writer. O padrão é '\r\n'.

Nota

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.

Dialect.quotechar

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 '"'.

Dialect.quoting

Controls when quotes should be generated by the writer and recognised by the reader. It can take on any of the QUOTE_* constants (see section Conteúdo do Módulo) and defaults to QUOTE_MINIMAL.

Dialect.skipinitialspace

When True, whitespace immediately following the delimiter is ignored. The default is False.

Dialect.strict

When True, raise exception Error on bad CSV input. The default is False.

13.1.3. Reader Objects

Reader objects (DictReader instances and objects returned by the reader() function) have the following public methods:

csvreader.next()

Return the next row of the reader’s iterable object as a list, parsed according to the current dialect.

Reader objects have the following public attributes:

csvreader.dialect

A read-only description of the dialect in use by the parser.

csvreader.line_num

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.

Novo na versão 2.5.

DictReader objects have the following public attribute:

csvreader.fieldnames

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.

Alterado na versão 2.6.

13.1.4. Objetos Writer

Writer objects (DictWriter instances and objects returned by the writer() function) have the following public methods. A row must be a sequence 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).

csvwriter.writerow(row)

Write the row parameter to the writer’s file object, formatted according to the current dialect.

csvwriter.writerows(rows)

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:

csvwriter.dialect

A read-only description of the dialect in use by the writer.

DictWriter objects have the following public method:

DictWriter.writeheader()

Write a row with the field names (as specified in the constructor).

Novo na versão 2.7.

13.1.5. Exemplos

The simplest example of reading a CSV file:

import csv
with open('some.csv', 'rb') as f:
    reader = csv.reader(f)
    for row in reader:
        print row

Reading a file with an alternate format:

import csv
with open('passwd', 'rb') 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', 'wb') as f:
    writer = csv.writer(f)
    writer.writerows(someiterable)

Registering a new dialect:

import csv
csv.register_dialect('unixpwd', delimiter=':', quoting=csv.QUOTE_NONE)
with open('passwd', 'rb') 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, 'rb') as f:
    reader = csv.reader(f)
    try:
        for row in reader:
            print row
    except csv.Error as e:
        sys.exit('file %s, line %d: %s' % (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

The csv module doesn’t directly support reading and writing Unicode, but it is 8-bit-clean save for some problems with ASCII NUL characters. So you can write functions or classes that handle the encoding and decoding for you as long as you avoid encodings like UTF-16 that use NULs. UTF-8 is recommended.

unicode_csv_reader() below is a generator that wraps csv.reader to handle Unicode CSV data (a list of Unicode strings). utf_8_encoder() is a generator that encodes the Unicode strings as UTF-8, one string (or row) at a time. The encoded strings are parsed by the CSV reader, and unicode_csv_reader() decodes the UTF-8-encoded cells back into Unicode:

import csv

def unicode_csv_reader(unicode_csv_data, dialect=csv.excel, **kwargs):
    # csv.py doesn't do Unicode; encode temporarily as UTF-8:
    csv_reader = csv.reader(utf_8_encoder(unicode_csv_data),
                            dialect=dialect, **kwargs)
    for row in csv_reader:
        # decode UTF-8 back to Unicode, cell by cell:
        yield [unicode(cell, 'utf-8') for cell in row]

def utf_8_encoder(unicode_csv_data):
    for line in unicode_csv_data:
        yield line.encode('utf-8')

For all other encodings the following UnicodeReader and UnicodeWriter classes can be used. They take an additional encoding parameter in their constructor and make sure that the data passes the real reader or writer encoded as UTF-8:

import csv, codecs, cStringIO

class UTF8Recoder:
    """
    Iterator that reads an encoded stream and reencodes the input to UTF-8
    """
    def __init__(self, f, encoding):
        self.reader = codecs.getreader(encoding)(f)

    def __iter__(self):
        return self

    def next(self):
        return self.reader.next().encode("utf-8")

class UnicodeReader:
    """
    A CSV reader which will iterate over lines in the CSV file "f",
    which is encoded in the given encoding.
    """

    def __init__(self, f, dialect=csv.excel, encoding="utf-8", **kwds):
        f = UTF8Recoder(f, encoding)
        self.reader = csv.reader(f, dialect=dialect, **kwds)

    def next(self):
        row = self.reader.next()
        return [unicode(s, "utf-8") for s in row]

    def __iter__(self):
        return self

class UnicodeWriter:
    """
    A CSV writer which will write rows to CSV file "f",
    which is encoded in the given encoding.
    """

    def __init__(self, f, dialect=csv.excel, encoding="utf-8", **kwds):
        # Redirect output to a queue
        self.queue = cStringIO.StringIO()
        self.writer = csv.writer(self.queue, dialect=dialect, **kwds)
        self.stream = f
        self.encoder = codecs.getincrementalencoder(encoding)()

    def writerow(self, row):
        self.writer.writerow([s.encode("utf-8") for s in row])
        # Fetch UTF-8 output from the queue ...
        data = self.queue.getvalue()
        data = data.decode("utf-8")
        # ... and reencode it into the target encoding
        data = self.encoder.encode(data)
        # write to the target stream
        self.stream.write(data)
        # empty queue
        self.queue.truncate(0)

    def writerows(self, rows):
        for row in rows:
            self.writerow(row)