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Files

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Diving In

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Reading From Text Files

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open(..., encoding='...')
open(..., 'r', encoding='...')

Character Encoding Rears Its Ugly Head

Bytes are bytes; characters are an abstraction. A string is a sequence of Unicode characters. But a file on disk is not a sequence of Unicode characters; a file on disk is a sequence of bytes. So if you read a “text file” from disk, how does Python convert that sequence of bytes into a sequence of characters? It decodes the bytes according to a specific character encoding algorithm, and returns a sequence of Unicode characters, otherwise known as a string.

# on Windows...
>>> file = open('examples/chinese.txt')
>>> a_string = file.read()
Traceback (most recent call last):
  File "", line 1, in 
  File "C:\Python31\lib\encodings\cp1252.py", line 23, in decode
    return codecs.charmap_decode(input,self.errors,decoding_table)[0]
UnicodeDecodeError: 'charmap' codec can't decode byte 0x8f in position 28: character maps to 
>>>

File Objects

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Python has a built-in function, open(), for opening a file on disk. The open() function returns a file object, which has methods and attributes for getting information about and manipulating the file.

Reading Data From A Text File

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6.2.2. Closing Files

Open files consume system resources, and depending on the file mode, other programs may not be able to access them. It’s important to close files as soon as you’re finished with them.

FIXME checking if a file is closed

Using The with Statement

FIXME "with open(...) as file" pattern

Reading Data One Line At A Time

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FIXME what's a "line"? (line endings discussion, universal line endings, etc.)

Writing to Text Files

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Character Encoding Again

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Write A Little, Write A Lot

FIXME write(), writelines(), .writeable

Handling I/O Errors

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Binary Files

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>>> image = open('examples/beauregard-100x100.jpg', 'rb')
>>> image
<io.BufferedReader object at 0x00C7A390>
>>> image.mode
'rb'
>>> image.name
'examples/beauregard-100x100.jpg'

>>> image
<io.BufferedReader object at 0x00C7A390>
>>> image.tell()
0
>>> data = image.read(3)
>>> data
b'\xff\xd8\xff'
>>> image.tell()
3
>>> image.seek(0)
0
>>> data = image.read()
>>> len(data)
3150

File-like Objects

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Standard Input, Output, and Error

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Further Reading

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© 2001–9 Mark Pilgrim