Files handling using python language.pptx

Files
• Most file methods are concerned with performing input
from and output to the external file associated with a file
object, but other file methods allow us to seek to a new
position in the file, flush output buffers, and so on. Table
summarizes common file operations.

Opening Files
• To open a file, a program calls the built-in open function, with
the external filename first, followed by a processing mode. The
call returns a file object, which in turn has methods for data
transfer:
• The first argument to open, the external filename, may include
a platform-specific and absolute or relative directory path
prefix. Without a directory path, the file is assumed to exist in
the current working directory (i.e., where the script runs).

• The second argument to open, processing mode, is typically the
string 'r' to open for text input (the default), 'w' to create and
open for text output, or 'a' to open for appending text to the
end (e.g., for adding to logfiles).
• The processing mode argument can specify additional options:
 • Adding a b to the mode string allows for binary data (end-of-
line translations and 3.X Unicode encodings are turned off).
• Adding a + opens the file for both input and output (i.e., you
can both read and write to the same file object, often in
conjunction with seek operations to reposition in the file).
Both of the first two arguments to open must be Python strings.
An optional third argument can be used to control output
buffering—passing a zero means that output is unbuffered

• Once you make a file object with open, you can call its
methods to read from or write to the associated external
file. In all cases, file text takes the form of strings in Python
programs; reading a file returns its content in strings, and
content is passed to the write methods as strings.
• File iterators are best for reading lines
• Content is strings, not objects: data read from a file always
comes back to your script as a string, so you’ll have to
convert it to a different type of Python object if a string is
not what you need.

• The following code begins by opening a new text file for
output, writing two lines (strings terminated with a newline
marker, n), and closing the file.

• If you want to display the file’s content with end-of-line
characters interpreted, read the entire file into a string all at
once with the file object’s read method and print it:

Storing Python Objects in Files:
Conversions
• convert objects to strings using conversion tools. Again, file
data is always strings in our scripts, and write methods do
not do any automatic to-string formatting for us

• the interactive echo gives the exact byte contents, while the
print operation interprets embedded end-of-line characters
to render a more user-friendly display:
We now have to use other conversion tools to translate from the
strings in the text file to real Python objects. As Python never converts
strings to numbers (or other types of objects) automatically, this is
required if we need to gain access to normal object tools like indexing,
addition, and so on:

For this first line, we used the string rstrip method to get rid of the trailing
end-of-line character; a line[: 1]
− slice would work, too, but only if we can be
sure all lines end in the n character (the last line in a file sometimes does
not).
We still must convert from strings to integers, though, if we wish to perform math on these:

int translates a string of digits into an integer object.
Notice that we didn’t have to run rstrip to delete the n at the end of the
last part; int and some other converters quietly ignore whitespace around
digits.
Finally, to convert the stored list and dictionary in the third line of the file,
we can run them through eval, a built-in function that treats a string as a
piece of executable program code

Storing Native Python Objects: pickle
• If you really want to store native Python objects, but you can’t trust the source of the data in
the file, Python’s standard library pickle module is ideal.
• The pickle module is a more advanced tool that allows us to store almost any Python object in
a file directly, with no to- or from-string conversion requirement on our part. It’s like a super-
general data formatting and parsing utility.
• To store a dictionary in a file, for instance, we pickle it directly:
Then, to get the dictionary back later, we simply use pickle again to re-create it:

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