Python basic data types
One step back: here we look at the built-in data types which are available without loading any modules.
Built-in data types and operations
There are four distinct numeric types:
plain integers int
long integers long
floating point numbers float
complex numbers complex
In addition, Booleans are a subtype of plain integers
Boolean
Boolean (logical) types
- True
- False
Operations
x + y sum of x and y
x - y difference of x and y
x * y product of x and y
x / y quotient of x and y
x // y (floored) quotient of x and y
x % y remainder of x / y
-x x negated
+x x unchanged
abs(x) absolute value or magnitude of x
int(x) x converted to integer
long(x) x converted to long integer
float(x) x converted to floating point
complex(re,im) a complex number with real part re,
imaginary part im. im defaults to zero
c.conjugate() conjugate of the complex number c
x ** y x to the power yBoolean Operations: and, or, not
x or y if x is false, then y, else x x and y if x is false, then x, else y not x if x is false, then True, else False
Comparisons
< strictly less than <= less than or equal > strictly greater than >= greater than or equal == equal != not equal is object identity
Sequence types
String str
List list
Tuple tuple
Operations
x in s True if an item of s is equal to x, else False
x not in s False if an item of s is equal to x,
else True
s + t the concatenation of s and t
s[i] i'th item of s
s[i:j] slice of s from i to j
s[i:j:k] slice of s from i to j with step k
len(s) length of s
min(s) smallest item of s
max(s) largest item of s
Variables, assignment
The assignment statement = creates new variables and gives them values
a=2 l=9**1000L S='Hello' f=1e5
Lists and tuples
Lists [] are mutable sequences of values
Tuples () can not be changed (immutable)
In [5]: a=[1,2,3] In [6]: b=(1,2,3) In [7]: a[0] Out[7]: 1 In [8]: b[0] Out[8]: 1 In [9]: a[0]=4 In [10]: a Out[10]: [4, 2, 3] In [11]: b[0]=4 --------------------------------------------------------------------------- exceptions.TypeError
Indexing and slicing
Indices [] select elements of variables
- Indices start from zero
x[a:b] selects a slice of elements from variable x
In [1]: a=range(10) In [2]: a Out[2]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] In [3]: a[0:4] Out[3]: [0, 1, 2, 3] In [4]: a[4:] Out[4]: [4, 5, 6, 7, 8, 9] In [5]: a[:4] Out[5]: [0, 1, 2, 3] In [6]: a[:-1] Out[6]: [0, 1, 2, 3, 4, 5, 6, 7, 8] In [7]: s='Hello world' In [8]: s[:5] Out[8]: 'Hello' In [9]: a=tuple(range(10)) In [10]: a[0:4] Out[10]: (0, 1, 2, 3)
List built-in functions (methods)
L.append(object) -- append object to end
L.count(value) -> integer -- return number of occurrences of value
L.extend(iterable) -- extend list by appending elements from the iterable
L.index(value, [start, [stop]]) -> integer -- return first index of value
L.insert(index, object) -- insert object before index
L.pop([index]) -> item -- remove and return item at index (default last)
L.reverse() -- reverse *IN PLACE*
L.sort() -- sort *IN PLACE*
In [72]: L=['C','A','B'] In [73]: L.sort() In [74]: L Out[74]: ['A', 'B', 'C'] In [75]: L.pop() Out[75]: 'C' In [76]: L Out[76]: ['A', 'B'] In [77]: L.insert(1,'C') In [78]: L Out[78]: ['A', 'C', 'B']
Nested data structures
In [84]: a=range(10) In [85]: b=range(0,10,2) In [86]: b Out[86]: [0, 2, 4, 6, 8] In [87]: c=[a,b] In [88]: c Out[88]: [[0, 1, 2, 3, 4, 5, 6, 7, 8, 9], [0, 2, 4, 6, 8]] In [91]: c[0] Out[91]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] In [92]: c[1] Out[92]: [0, 2, 4, 6, 8] In [93]: c[0][0:4] Out[93]: [0, 1, 2, 3] In [94]: c[1][0:4] Out[94]: [0, 2, 4, 6]
Dictionary
- A dictionary is like a list, but more general.
- In a list, the indices have to be integers; in a dictionary they can be (almost) any type.
d={} creates empty dictionary
d[key]=value add a key, value pair to dictionary
d.keys() returns list of keys
In [95]: D={}
In [96]: D['a']=range(10)
In [97]: D['b']=range(0,10,2)
In [98]: D
Out[98]: {'a': [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], 'b': [0, 2, 4, 6, 8]}
In [99]: D['a']
Out[99]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
In [100]: D.keys()
Out[100]: ['a', 'b']
Loops
Looping over a list is done for the for statement
enumerate() returns an enumerated object
In [106]: a=['Hund','Katze','Maus'] In [108]: for i in a: .....: print i .....: Hund Katze Maus In [111]: for i,j in enumerate(a): .....: print i,j .....: 0 Hund 1 Katze 2 Maus
Control statements
Conditions can be checked with if-else statement
if condition: indented block
else:
In [116]: if a==1: .....: print 'a=1' .....: else: .....: print 'a != 1' .....: a=1