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# The "hash-mark" indicates the start of a comment
# Python automatically assigns the right type:
a = 3 # Another Comment
b = 1.1
c = 4 + 6j
print(type(a), type(b), type(c))
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# Mixing types in an arithmetic expression
# will yield the "wider" type
print(b * 5, type(b * 5))
print (b + c, type(b + c))
Integer division yields integer!
float())
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print(a / 4, a / 4.0, float(a) / 4)
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# The Boolean Type: `bool`
test = 3 > 4
tost = 5 != 9
print(test, type(test))
print(tost, type(tost))
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# More Information about numeric datatypes
from IPython.display import HTML
HTML('<iframe src="https://docs.python.org/2/library/stdtypes.html#numeric-types-int-float-long-complex" width=700 height=350></iframe>')
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") and single quotes (') are synonymous.''') preserve line endings.\) is the escape-character.
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s1 = 'He\'s a lumberjack, and he\'s okay.\nHe sleeps all night and he works all day.'
s2 = '''I cut down trees. I eat my lunch.
I go to the lavatory.
On Wednesdays I go shoppin'
And have buttered scones for tea.'''
print(s1)
print
print(s2)
+"format() - method
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('{:.4f} is a {}'.format(pi,'float') +
' and "a = {}" needs no formatting in excercise {:02d}.'.format(3.1,1))
# Btw. : line-breaks inside brackets are implicitly continued
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# More on the format-specification
from IPython.display import HTML
HTML('<iframe src="https://docs.python.org/2/library/string.html#format-specification-mini-language" width=700 height=350></iframe>')
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s = "0123456789"
index0 = s[0]
index9 = s[9]
print("Index zero: {}, index nine: {}".format(index0, index9))
Note:
i retuns the (i+1)st character.
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# Negative indices count from the end
# Slice using "[start:stop:step]"
# Any of "start", "stop", "step" can be omitted
# They default to 0, len(string), 1, respectively
s = "0123456789"
print(s[-1], s[-2], s[0:2], s[0:9], s[0:10], s[:], s[:3], s[3:],s[::2])
Note: Strings are "immutable"
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s = "0123456789"
s[0] = 1
There are a largne number of string methods.
Examples:
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'Py' in 'Oython'
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'Python'.find('tho')
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s.isdigit()
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words = array("Words usually are obtained by splitting at whitespace".split())
nowords = "Words usually are obtained by splitting at whitespace".split('a')
print(words)
print(nowords)
type(words)
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'_'.join(words)
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Ipython Trick
Get a list of all available methods for an object by writing "objectname." and hitting [TAB].
Try "words.[TAB]"
Also "str.[TAB]"
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# Documentation of string methods
from IPython.display import HTML
HTML('<iframe src="https://docs.python.org/2/library/stdtypes.html#string-methods" width=700 height=350></iframe>')
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L = ['red', 2, 'blue', 3, 'green', 9.9, 'black', 0.7, 'white', -5]
type(L)
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# Indexing and slicing just as for strings
print(L[0])
print(L[-1:3:-1])
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# Reverse a list
L[::-1]
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Note: Lists are "mutable"
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L = ['red', 2, 'blue', 3, 'green', 9.9, 'black', 0.7, 'white', -5]
L[2:6] = ['violet',4,'turquois',10.1]
L
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# removing elements
del(L[4:6])
L
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# remove an get last element
L = ['red', 2, 'blue', 3, 'green', 9.9, 'black', 0.7, 'white', -5]
print(L)
l1 = L.pop()
print(l1)
print(L)
print(L.sort())
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# insert element at index
L.insert(3,'between3, blue and 3')
print(L)
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# append and extend
L = ['red', 2, 'blue', 3, 'green', 9.9, 'black', 0.7, 'white', -5]
print('original:{}'.format(L))
L1 = [1,'other','list']
L.append(L1)
print('append: {}'.format(L))
L.extend(L1)
print('extend:{}'.format(L))
L = L + L1
print('concatenated: {}'.format(L))
L.extend(L1)
VERY USEFUL: List comprehension
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x = range(0,10)
# apply to each element
y2 = [y**2 for y in x]
# complicated functions can be applied
x3 = [y/2.0 if y >= 25 else sqrt(y) for y in x2]
# filter
x4 = [y for y in x3 if y % 2 == 0]
print(x)
print(x2)
print(x3)
print(x4)
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t = ('hello',1,'triple')
print(type(t))
t
print(type([t]))
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s = set(("a",4,"b",5))
print(type(s))
s1 = set((7,5,"c","b",7,"b"))
union = s | s1
intersect = s & s1
print("s1: {}".format(s1))
print("union: {}".format(union))
print("intersect: {}").format(intersect)
l1 = list(s1)
print(l1)
l1.sort()
print(l1)
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<span style="background-color:grey" # 5 Minutes to play with lists, sets and tuples</span>
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numbers = { "jack": 04377465, "john": 97264384, "ralph":['+41 76 873274',937283] }
print(type(numbers))
numbers
# Observe how Jack's number get interpreted (as octal)
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numbers = { "jack": 04377465, "john": 97264384, "ralph":['+41 76 873274',937283] }
print(numbers)
print(numbers["john"]) # access by name
print(numbers["ralph"][0]) # concatenate accessors for contained objects
numbers["jenny"] = 9374 # add new entries
del(numbers["jack"]) # delete entries
numbers
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# "in" and "not in" work for all containers
print('ralph' in numbers)
print('ralph' not in numbers)
print(97264384 in numbers.values())
[str(x) + 'MODIFIED' for x in numbers.values()]
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# Another way to construct dictionaries:
d = dict([(1,"one"), (2,"two"), (3,"three"), (4,"four")])
d
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# From lists to dictionaries
l1 = ["These","are","words","in","a","sentence"]
l2 = range(0,len(l1))
# Note: "len()" gives the number of elements of any container type
# zip creates a list of corresponding pairs of two lists with equal length
pairs = zip(l1,l2)
dicty = dict(pairs)
print("l1 : {}".format(l1))
print("l2 : {}".format(l2))
print("pairs : {}".format(pairs))
print("dicty : {}".format(dicty))
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# There are more dictionary methods
from IPython.display import HTML
HTML('<iframe src="https://docs.python.org/2/library/stdtypes.html#dict" width=700 height=350></iframe>')
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n = 23
a = range(0,11)
if n in a:
print("In")
elif n == 23:
print("Should be in")
else:
print("{} is a bad number.".format(n))
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# The "ternary operator"
n = 2
a = range(0,11)
x = "In" if n in a else "Bad number"
print(x)
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# for
for i in range(20,31):
print(i**2),
# Note: The comma at the end of the "print" statement suppress printing of a newline at the end.
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# while
q = 10
while q <= 900:
q += q*3
print(q),
Reminder: Use list-comprehension if possible!
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# dict.iteritems()
# iterating over keys *and* values of dictionaries
d = dict([(1,"one"), (2,"two"), (3,"three"), (4,"four")])
for k, v in d.iteritems():
print("Key: {}, Value: {}".format(k, v))
# Note a tuple T (or list) on the right hand side of an assignment
# can be directly assigned to len(T) individual variables on the left hand side.
a, b, c, d = (1,2,3,4)
print(a,b,c,d)
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# enumerate(list)
# iterate over list and list-index at the same time
l1 = ["These","are","words","in","a","sentence"]
for i, x in enumerate(l1):
print("Index = {}; Item = {}".format(i,x))
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# Function definition
def my_function(a, b, c):
print(a, b, c)
res = (a + b)**c
return(res)
my_function(2, 3, 1.123)
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There are required positional- and optional keyword- parameters (just like in R)
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def my_func2(a, b, default=1):
print(a, b, default)
res = (a + b)**default
return(res)
print(my_func2(2,3))
print(my_func2(2, 3, 1.123))
In the next session, we need several files from the ERA-Interim reanalysis dataset.
Write a function that takes year, month, startday, and endday as parameters, and returns
list a of corresponding Z-level filenames (with absolute paths).
The input parameters have to be numeric (not string).
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# The path to erainterim
root = '/net/atmos/data/erainterim/cdf'
# below that directory the path is <year>/<month>/.
# <year> has values from 1979 - 2014, <month> from 01 - 12.
# Under the <month> directory is a large number of files.
# The Z-files have the form:
# Z<year><month><day>_hour
# For example:
# <root>/2010/09/Z20100903_18
# refers to Sept. 3, 2010, 18:00h
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# helpful modules
import os.path
import glob
# example parameters
startday = 3
endday = 8
month = 9
year = 2010
# your function here:
files = get_zfile(startday, endday, month, year)
print(files)
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