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locals, globals)
Like exec() (see above) but
operates on a named file.
Dynamically reads and
executes the Python code in
the file. Allows optional
dictionary arguments to
specify global and local
scope variable names. Not
available in Python 3.
execfile(fileName)
file(name, mode)
Creates a file object. Little
if isinstance(obj,
used because open(),
described below, is generally
used instead, but useful for
checking data types. Not
available in Python 3.
file):
print("Object is a
file")
float( val)
Converts a number or string
into its best floating point
equivalent.
print(float(‘7’))
print(float(‘+18.57e12’) /
2.0)
format(val,
specification)
Formats a string value
according to a specified
representation. See
Appendix
4
below for new-style
formatting codes.
format(3.14159,‘E’)
format(‘DNA’, ‘>4s’)
Results are ‘3.141590E+00’
and ‘DNA’.
frozenset(vals)
Creates a frozen set from a
given object; an immutable
unordered collection of non-
repeating items. Sometimes
used to convert sets into the
immutable equivalent so they
may be used as keys in
dictionaries.
l = [0,1,2,3,1,2,3,2,1,0,1,3]
x = frozenset(l)
print(x)
Result is frozenset([0, 1, 2,
3]).
getattr(obj, name,
defaultVal)
Retrieves a particular named
attribute from a specified
Python object. Sometimes
used as an alternative to the
dot notation when the
attribute name is a variable.
import math
print(getattr(math,
'pi'))
print(math.pi)
Results are both
3.141592653589793.
globals()
Gives a dictionary of variable
name and object pairs that are
available in the outermost,
global context of the
program’s execution.
varDict = globals()
print(varDict)
hasattr(obj, name)
Determines whether a Python
object has a particular named
attribute. Gives True if it
does and False otherwise.
import math
hasattr(math,'sin') #
True
hasattr(math,'apple')
# False
help( obj)
Gets help documentation for
a given Python object.
print(help(float))
fileObj = open(‘a.txt’)
print(help(fileObj))
hex(val)
Creates a text string
representing the hexadecimal
(i.e. base 16) version of an
integer number.
print(hex(2**24-1))
Result is ‘0xffffff’.
id(obj)
Gives the unique number that
identifies a particular Python
object. Such numbers do not
change within a Python
session, but they will
(usually) be different for a
new session.
print(id(7))
print(id(float))
print(id(None))
input(prompt)
Prompts the user at the
command line for keyboard
input, i.e. so the entered
values can be used in the
program.
x = input(‘Enter value:’)
print(“Value is: “, x)
int(val, base)
Converts a number or string
into an integer representation.
Optionally takes the radix
number for which base to
use.
print(int(34.96)) # Base 10
print(int(‘1000100’,2)) #
Base 2
Results are 34 and 68.
isinstance(obj,
class)
Determines whether one
object is derived from a
specified class (or subclass
thereof), returning True if it
is and False otherwise.
isinstance(7, int) # True
isinstance(7, float) # False
issubclass(class1,
class2)
Determines whether one
object class is a subclass of
another, returning True if it is
and False otherwise.
issubclass(Protein,
Molecule)
len(vals)
Gives the size of an object,
e.g. the number of items in a
collection.
letters = [‘G’,‘C’,‘A’,‘T’]
print(len(letters)) # 4
list(vals)
Creates a Python list
t = (‘V’,‘I’,‘L’,‘A’,‘M’,‘P’)
collection from a specified
object, which must be
iterable. Can be used on a list
to make a copy.
x = list(t)
y =
list(enumerate(range(7,11)))
First example converts
tuple to list. Second
converts iterator, giving [(0,
7), (1, 8), (2, 9), (3, 10)].
locals()
Gives a dictionary of variable
name and object pairs that are
available in the innermost,
local context at a point in the
program’s execution.
for x in range(5):
varDict = locals()
print(varDict)
Results show x changes
locally, in the loop.
long(val, base)
Converts val into a long
integer (i.e. of arbitrary
length): this is mostly
redundant. Not available in
Python 3.
[See Mathematics section]
map(func, vals,
…)
Takes an iterable object, like
a list, and applies a function
to each item, generating a
new list (in Python 2) or a
map iterator (in Python 3).
This function is largely
redundant and it is more
commonplace to use a list
comprehension instead.
vals = [30.0, 31.0, 28.25]
ints1 = map(int, vals)
ints2 = [int(x) for x in vals]
Note: ints1 and ints2 are the
same in Python 2, but in
Python 3 ints1 is a map
iterator.
max(vals)
or
max(val1, val2,
…)
Finds the minimum value of
a collection (or other iterable
object) or list of arguments.
This does not work on multi-
dimensional NumPy arrays.
print(max(3,11, 9, 5))
# 11
l = [30.0, 31.0, 28.25]
print(max(l)) # 31.0
min(vals)
or
min(val1, val2,
…)
Finds the minimum value of
a collection (or other iterable
object) or list of arguments.
This does not work on multi-
dimensional NumPy arrays.
print(min(8, 12, 3, 34)) # 3
l = [30.0, 31.0, 28.25]
print(min(l)) # 28.25
object()
Creates a basic, blank,
featureless object. The class
x = object()
print(dir(x))
of the object is the superclass
of all Python objects.
oct(val)
Creates a text string
representing the octal (i.e.
base 8) version of an integer
number.
print(oct(2**24-1))
Result is ‘077777777’ in
Python 2 and ‘0o77777777’
in Python 3.
open(fileName,
mode)
Opens a file on disk for
reading or writing by creating
a file type object. The second
argument is a mode string
that specifies whether the file
is for reading (‘r’), writing
(‘w’), appending (‘a’) etc.
The default mode is reading.
fObj1 = open(inName,
'rU')
line = fObj1.readline()
fObj2 = open(outName,
'w')
fObj2.write('Hello
world\n')
ord(char)
Gives the ASCII code
number for a character string.
Performs the inverse
operation to chr().
i = ord(‘Z’)-1
print(i, chr(i))
Result is 89, Y.
pow(val1, val2,
modulo)
Raises a number to a given
power, equivalent to using
the ‘**’ operator.
x = pow(2, 8)
x = 2**8 # Same
print(text, sep,
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