Python Crash Course
File I/O
3rd year Bachelors
V1.0
dd 04-09-2013
Hour 1
File I/O
• Types of input/output available
– Interactive
• Keyboard
• Screen
– Files
• Ascii/text
– txt
– csv
• Binary
• Structured
– FITS > pyFITS, astropy.io.fits
• URL
• Pipes
Interactive I/O, fancy output
>>> s = 'Hello, world.'
>>> str(s)
'Hello, world.'
>>> repr(s)
"'Hello, world.'"
>>> str(1.0/7.0)
'0.142857142857'
>>> repr(1.0/7.0)
'0.14285714285714285'
>>> x = 10 * 3.25
>>> y = 200 * 200
>>> s = 'The value of x is ' + repr(x) + ', and y is ' + repr(y) + '...'
>>> print s
The value of x is 32.5, and y is 40000...
>>> # The repr() of a string adds string quotes and backslashes:
... hello = 'hello, world\n'
>>> hellos = repr(hello)
>>> print hellos
'hello, world\n'
>>> # The argument to repr() may be any Python object:
... repr((x, y, ('spam', 'eggs')))
"(32.5, 40000, ('spam', 'eggs'))"
Interactive I/O, fancy output
Old string formatting
>>> import math
>>> print 'The value of PI is approximately %5.3f.' % math.pi
The value of PI is approximately 3.142.
New string formatting
>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
>>> for name, phone in table.items():
...
print '{0:10} ==> {1:10d}'.format(name, phone)
...
Jack
==>
4098
Dcab
==>
7678
Sjoerd
==>
4127
Formatting I/O
A conversion specifier contains two or more characters and has the following
components, which must occur in this order:
•
•
•
•
•
•
•
The "%" character, which marks the start of the specifier.
Mapping key (optional), consisting of a parenthesised sequence of characters (for example,
(somename)).
Conversion flags (optional), which affect the result of some conversion types.
Minimum field width (optional). If specified as an "*" (asterisk), the actual width is read from the
next element of the tuple in values, and the object to convert comes after the minimum field width
and optional precision.
Precision (optional), given as a "." (dot) followed by the precision. If specified as "*" (an asterisk),
the actual width is read from the next element of the tuple in values, and the value to convert
comes after the precision.
Length modifier (optional).
Conversion type.
>>> print '%(language)s has %(#)03d quote types.' % \
{'language': "Python", "#": 2}
Python has 002 quote types.
The conversion types are:
Conversion
Meaning
d
Signed integer decimal.
i
Signed integer decimal.
o
Unsigned octal.
u
Unsigned decimal.
x
Unsigned hexadecimal (lowercase).
X
Unsigned hexadecimal (uppercase).
e
Floating point exponential format (lowercase).
E
Floating point exponential format (uppercase).
f
Floating point decimal format.
F
Floating point decimal format.
g
Same as "e" if exponent is greater than -4 or less than precision, "f" otherwise.
G
Same as "E" if exponent is greater than -4 or less than precision, "F" otherwise.
c
Single character (accepts integer or single character string).
r
String (converts any python object using repr()).
s
String (converts any python object using str()).
%
No argument is converted, results in a "%" character in the result.
Interactive I/O
>>> print “Python is great,”, ”isn’t it?”
>>> str = raw_input( “Enter your input: ”)
>>> print “Received input is: “,str
Enter your input: Hello Python
Received input is: Hello Python
>>> str = input("Enter your input: ");
>>> print "Received input is: ", str
Enter your input: [x*5 for x in range(2,10,2)]
Received input is: [10, 20, 30, 40]
If the readline modules was loaded the raw_input() will use it to provide elaborate line editing and
history features.
File I/O
>>> fname = ‘myfile.dat’
>>> f = file(fname)
>>> lines = f.readlines()
>>> f.close()
>>> f = file(fname)
>>> firstline = f.readline()
>>> secondline = f.readline()
>>> f = file(fname)
>>> for l in f:
...
print l.split()[1]
>>> f.close()
>>>
>>>
>>>
>>>
outfname = ‘myoutput’
outf = file(outfname, ‘w’) # second argument denotes writable
outf.write(‘My very own file\n’)
outf.close()
Read File I/O
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
f = open("test.txt")
# Read everything into single string:
content = f.read()
len(content)
print content
f.read() # At End Of File
f.close()
# f.read(20) reads (at most) 20 bytes
Using with block:
>>> with open(’test.txt’, ’r’) as f:
...
content = f.read()
>>> f.closed
CSV file:
>>>
>>>
>>>
>>>
...
>>>
import csv
ifile = open(’photoz.csv’, "r")
reader = csv.reader(ifile)
for row in reader:
print row,
ifile.close()
Read and write text file
>>> from numpy import *
>>> data = loadtxt("myfile.txt") # myfile.txt contains 4 columns of numbers
>>> t,z = data[:,0], data[:,3] # data is a 2D numpy array, t is 1st col, z is 4th col
>>> t,x,y,z = loadtxt("myfile.txt", unpack=True) # to automatically unpack all columns
>>> t,z = loadtxt("myfile.txt", usecols = (0,3), unpack=True) # to select just a few columns
>>>
>>>
>>>
>>>
data
data
data
data
=
=
=
=
loadtxt("myfile.txt",
loadtxt("myfile.txt",
loadtxt("myfile.txt",
loadtxt("myfile.txt",
>>>
>>>
>>>
>>>
>>>
>>>
>>>
from numpy import *
savetxt("myfile.txt",
savetxt("myfile.txt",
savetxt("myfile.txt",
savetxt("myfile.txt",
savetxt("myfile.txt",
savetxt("myfile.txt",
skiprows = 7) # to skip 7 rows from top of file
comments = '!') # use '!' as comment char instead of '#'
delimiter=';') # use ';' as column separator instead of whitespace
dtype = int) # file contains integers instead of floats
data) # data is 2D array
x) # if x is 1D array then get 1 column in file.
(x,y)) # x,y are 1D arrays. 2 rows in file.
transpose((x,y))) # x,y are 1D arrays. 2 columns in file.
transpose((x,y)), fmt='%6.3f') # use new format instead of '%.18e'
data, delimiter = ';') # use ';' to separate columns instead of space
String formatting for output
>>> sigma = 6.76/2.354
>>> print(‘sigma is %5.3f metres’%sigma)
sigma is 2.872 metres
>>> d = {‘bob’: 1.87, ‘fred’: 1.768}
>>> for name, height in d.items():
...
print(‘%s is %.2f metres tall’%(name.capitalize(), height))
...
Bob is 1.87 metres tall
Fred is 1.77 metres tall
>>>
>>>
>>>
>>>
...
...
>>>
nsweets = range(100)
calories = [i * 2.345 for i in nsweets]
fout = file(‘sweetinfo.txt’, ‘w’)
for i in range(nsweets):
fout.write(‘%5i %8.3f\n’%(nsweets[i], calories[i]))
fout.close()
File I/O, CSV files
• CSV (Comma Separated Values) format is the most common import
and export format for spreadsheets and databases.
• Functions
–
–
–
–
–
–
–
csv.reader
csv.writer
csv.register_dialect
csv.unregister_dialect
csv.get_dialect
csv.list_dialects
csv.field_size_limit
File I/O, CSV files
•
Reading CSV files
import csv
# imports the csv module
f = open('data1.csv', 'rb')
try:
reader = csv.reader(f)
for row in reader:
print row
finally:
f.close()
# opens the csv file
•
# creates the reader object
# iterates the rows of the file in orders
# prints each row
# closing
Writing CSV files
import csv
ifile
reader
ofile
writer
=
=
=
=
open('test.csv', "rb")
csv.reader(ifile)
open('ttest.csv', "wb")
csv.writer(ofile, delimiter='\t', quotechar='"', quoting=csv.QUOTE_ALL)
for row in reader:
writer.writerow(row)
ifile.close()
ofile.close()
File I/O, CSV files
• The csv module contains a the following quoting options.
• csv.QUOTE_ALL
Quote everything, regardless of type.
• csv.QUOTE_MINIMAL
Quote fields with special characters
• csv.QUOTE_NONNUMERIC
Quote all fields that are not integers or floats
• csv.QUOTE_NONE
Do not quote anything on output
File I/O, Pickle
• Pickle: powerful algorithm for serializing and de-serializing a Python
object structure
–
–
–
–
can transform a complex object into a byte stream
can transform the byte stream into an object with the same internal structure
most obvious thing to do with these byte streams is to write them onto a file
also conceivable to send them across a network or store them in a database
• The following types can be pickled:
–
–
–
–
–
–
–
–
None, True, and False
integers, long integers, floating point numbers, complex numbers
normal and Unicode strings
tuples, lists, sets, and dictionaries containing only picklable objects
functions defined at the top level of a module
built-in functions defined at the top level of a module
classes that are defined at the top level of a module
instances of such classes whose __dict__ or the result of calling __getstate__()
is picklable (see section The pickle protocol for details).
File I/O, Pickle
• Example save
import pickle
data1 = {'a': [1, 2.0, 3, 4+6j],
'b': ('string', u'Unicode string'),
'c': None}
selfref_list = [1, 2, 3]
selfref_list.append(selfref_list)
output = open('data.pkl', 'wb')
# Pickle dictionary using protocol 0.
pickle.dump(data1, output)
# Pickle the list using the highest protocol available.
pickle.dump(selfref_list, output, -1)
output.close()
File I/O, Pickle
• Example load
import pprint, pickle
pkl_file = open('data.pkl', 'rb')
data1 = pickle.load(pkl_file)
pprint.pprint(data1)
data2 = pickle.load(pkl_file)
pprint.pprint(data2)
pkl_file.close()
(dp0
S'a'
p1
(lp2
I1
aF2.0
aI3
ac__builtin__
complex
p3
(F4.0
F6.0
tp4
Rp5
asS'c'
p6
NsS'b'
p7
(S'string'
p8
VUnicode string
p9
tp10
s.� ]q
File I/O, Pickle
• Real live example AstroWise cluster job submission
– client – server model
– exchanging code & data for remote processing
def dpu_packit(*args):
return pickle.dumps(args)
def dpu_unpackit(data):
return pickle.loads(data)
# Sender side:
def submitremotejobs(self, key, zip=None, jobs=[], env=None):
if not len(jobs): return False
return self.senddata(key, dpu_packit((zip, env), jobs))
# Receiver side:
data = self.get_data()
((code, env), jobdictlist) = dpu_unpackit(data)
make_code_file(key, code)
URL
•
URLS can be used for reading
>>> import urllib2
>>> url = 'http://python4astronomers.github.com/_downloads/data.txt'
>>> response = urllib2.urlopen(url)
>>> data = response.read()
>>> print data
RAJ
DEJ
Jmag
e_Jmag
2000 (deg) 2000 (deg) 2MASS
(mag) (mag)
---------- ---------- ----------------- ------ -----010.684737 +41.269035 00424433+4116085
9.453 0.052
010.683469 +41.268585 00424403+4116069
9.321 0.022
010.685657 +41.269550 00424455+4116103 10.773 0.069
010.686026 +41.269226 00424464+4116092
9.299 0.063
010.683465 +41.269676 00424403+4116108 11.507 0.056
010.686015 +41.269630 00424464+4116106
9.399 0.045
010.685270 +41.267124 00424446+4116016 12.070 0.035
URL
•
URLS sometimes need input data. Such as POST data for a form
import urllib
import urllib2
url = 'http://www.someserver.com/cgi-bin/register.cgi'
values = {'name' : 'Michael Foord',
'location' : 'Northampton',
'language' : 'Python' }
data = urllib.urlencode(values)
req = urllib2.Request(url, data)
response = urllib2.urlopen(req)
the_page = response.read()
URL
•
And for GET type of parameter passing:
import urllib
import urllib2>>> import urllib2
>>> import urllib
>>> data = {}
>>> data['name'] = 'Somebody Here'
>>> data['location'] = 'Northampton'
>>> data['language'] = 'Python'
>>> url_values = urllib.urlencode(data)
>>> print url_values # The order may differ.
name=Somebody+Here&language=Python&location=Northampton
>>> url = 'http://www.example.com/example.cgi'
>>> full_url = url + '?' + url_values
>>> handler = urllib2.urlopen(full_url)
Note that the full URL is created by adding a ? to the URL, followed by the encoded
values.
Introduction to language
End