Table of Contents
Python in a nutshell
Python is a powerful scripting language allowing the creation of input files in a very convenient way for the developer, but it might look a bit tedious for a new user.
When Metafor is started, a window appears:
This window displays the command-line interpreter. This is a Python interpreter integrated within Metafor, allowing the user to type commands, in a similar way as in MATLAB.
Above the interpreter, a list of all the restrictions associated with the executable is shown (time limit, maximal number of nodes, …). The figure above corresponds to a full version associated to a specific computer.
Once Python is initialized, Metafor automatically loads the module
toolbox.utilities, which contains a set of useful commands. To be more accurate, only the most useful functions of this module are loaded, to avoid using too much space in the root namespace.
A Metafor input file is a series of Python commands. An obvious disadvantage of a python script is that it is less user-friendly than graphical interfaces of Samcef or Abaqus CAE. However, in this context all Python tools (functions, objects, loops, conditional structures, …) can be used to easily create and parametrize a problem, leading to a very efficient way of developing numerical models.
Another interesting aspect of Python is its ability to interface compiled C++ code without the help of the programmer (the modules
wrap are automatically created from the C++ source code). Therefore, Python is ideal for both the developer and the (well-informed) user.
Python is quite simple. For example, it can be used as a calculator in interactive mode:
>>> a=2 >>> b=3 >>> c=a+b >>> c 5 >>>
In a script, the correct syntax to get the value of
c requires the command
The definition of variables allows the parametrization of Metafor input files. Therefore, as with every other piece of software, it is important to first define a set of variables and then work with these algebraic variables.
To display memory contents, use
>>>> dir() ['__builtins__', '__doc__', '__name__', 'a', 'b', 'c', 'getDomain', 'getPath', 'loadFac', 'makeAnimation', 'meta', 'quit', 'restart', 'vizu'] >>>
The three variables that were just created can be seen, and so can the
toolbox.utilities functions that were automatically imported (
vizu, …). Finally,
_ is a module containing basic Python functions. To see them, the function
dir() is used again.
>>> dir(__builtins__) ['ArithmeticError', 'AssertionError', 'AttributeError', 'DeprecationWarning', 'EOFError', 'Ellipsis', 'EnvironmentError', 'Exception', 'False', 'FloatingPointError', 'FutureWarning', 'IOError', 'ImportError', 'IndentationError', 'IndexError ...
The type of a variable is obtained with the command
type(var). Every variable has a type (also called a “class”), however in Python the type of a variable can change during the execution of the code, unlike in Fortran or C.
>>> a=4 >>> type(a) <class 'int'> >>> a='text' >>> type(a) <class 'str'> >>>
Metafor uses the basic python types (float, string, integer, tuple, …), but also its own types, which are defined by custom classes.
Finally, variables can be converted from one type to another. For example,
int(2.3) is 2, when
str(2.3) gives the string “2.3”.
Simple functions can be created using the interpreter. This is quite useful to avoid any cut and paste, or simply to write algorithms as scripts. For example:
>>> def square(x): return x*x ... >>> square(6) 36 >>>
Functions are essential in the proper writing of an input file. This way, complex operations may be defined and called several times, from the same module or even from other ones. For example, if the numerical model contains several similar contact matrices, it is worth parametrizing the creation of a matrix (definition of its points, lines, …), and then calling it several times with different arguments. Materials may also be assigned using functions.
Default values for the parameters of python functions may be set:
def myfun(x, a=1, b=1, c=0): return a*x*x+b*x+c
c are optional parameters. For instance, this function may be called with the command :
y = myfun(3, b=2)
This function may be written in another way, using the
lambda functions :
myfun = lambda x, a=1, b=1, c=0 : a*x*x+b*x+c
It is quite useful to gather several functions into a file, to form a module (one module is defined with one file). In Metafor, every test case is defined in a module. For example, the test case named
cont2 is located in
Metafor/apps/qs/cont2.py and identified by
apps.qs.cont2 under Python.
By default, Python already contains many modules. For example, the module called
math defines all basic mathematical functions. To use a module, the statement
import must be used (for example,
import math). Then, to use a function of the
math module, the point is used. For example,
math.cos is the command used to compute a cosine:
>> import math >>> dir(math) ['__doc__', '__loader__', '__name__', '__package__', '__spec__', 'acos', 'acosh', 'asin', 'asinh', 'atan', 'atan2', 'atanh', 'ceil', 'copysign', 'cos', 'cosh', 'degrees', 'e', 'erf', 'erfc', 'exp', 'expm1', 'fabs', 'factorial', 'floor', ... 'sqrt', 'tan', 'tanh', 'tau', 'trunc'] >>> math.cos(0.1) 0.99500416527802582 >>>
The online help is accessed with the command
>>> help(math.cos) Help on built-in function cos in module math: cos(...) cos(x) Return the cosine of x (measured in radians).
help(math) enumerates all functions found in the
math module. The online help is quite useful and also available for all Metafor modules.
It is also possible to import all functions found in
math in the current namespace with the command:
>>> from math import * >>> cos(0.1) 0.99500416527802582 >>>
With this syntax, the
math prefix is no longer necessary. However, conflicts could occur with other modules if they also contain a function named
To use mathematical functions, the module
math must be imported. In the same way, the functions which are used to build an input file in Metafor must be imported from the module
wrap, using the command
from wrap import *.
Python is object-oriented. This means that classes can be created. To make it short, a class is a structure containing data, to which methods can be associated. For example, to compute the length of a vector, a function can be defined :
def vectLength(x, y, z): import math return math.sqrt(x*x + y*y + z*z)
The function called
vectLength computes the length of the vector
(x,y,z). However, this is not object-oriented, because the encapsulation principle is violated (the user is directly manipulating the components). In order to have an object-oriented code, a class (so a new type) representing this vector is created:
class vector: def __init__(self,x,y,z): self.x = x self.y = y self.z = z def view(self): print('x=', self.x, ' y=', self.y, ' z=', self.z) def length(self): import math return math.sqrt(self.x*self.x + self.y*self.y + self.z*self.z)
Three methods (or member functions) have been defined:
_, the constructor (see below) ;
view(), displaying the vector using the function
length, returning the length. Each method is defined within the class (see the indentation).
To compute the length of a vector, this vector is first created using the command
>>> v = vector(1, 1, 1)
This command calls the constructor (method
_) of the class named
vector. It should be noted that this function is defined with four parameters, the first one being
self (equivalent to the pointer
this in C++), but called with only three parameters (the
self pointer is passed implicitly).
>>> type(v) <class '__main__.vector'> >>> v.view() x= 1 y= 1 z= 1 >>> v.length() 1.7320508075688772 >>>
When creating an input file in Metafor, the user manipulates classes defined in the modules found in the folder
wrap. Therefore, the user should understand basic object-oriented programming, even if an input file may be written without defining new objects.
Loop - tuples - lists
In a Metafor input file, the same operation may be used several times in a row (for example, meshing the curves numbered from 1 to 10). In such a case, loops are used. A loop is written as:
for i in (1, 6, 8, 12): print('i=', i)
where the values in parentheses constitute a “tuple”. In this example, “i” will successfully take the values 1, 6, 8 and 12, leading to four executions of the command
Instead of tuples, lists may also be used. To do so, brackets are used instead of parentheses. Unlike tuples, lists may be modified.
To create a list, the function
range(i, j, k) is quite often used. This function works as
i:k:j in Matlab.
>>> list(range(1, 10, 2)) [1, 3, 5, 7, 9]
A list may also be defined as :
[ x for x in range(100) if x%3==0 ]
leading to all numbers from 0 to 99 which can be divided by 3.
It may be useful to parametrize an input file in a complex way using conditional statements. For example, a boolean
useLinearMaterial can be defined, to choose whether the material behaves in an linear way. This parameter, defined in the beginning of the input file, is easy to modify and leads to a conditional definition of a linear material. Such a construct requires the command
if, which is used as :
if x>100: print ("x>100") elif x>10: print ("x>10") else: print ("x<=10")
Falsebooleans are noted with capital letter.
- A number starting with 0 corresponds to a number defined using the octal numeral system. For example, 012 corresponds to 10 in decimal.
- The command
input('message')is used to ask the user to enter a value. This is useful to pause the input file, for example to view an intermediate mesh.
- The function
print(a, b, c)prints the objects
con the same line, which can be quite useful for Metafor objects.
- The null command is named
- Several commands may be written on the same line if separated by a semicolon, as in C.
- Strings are defined with simple ('), double (“) or triple quotation marks (”“”).
In this page, the basic use of Python in a Metafor context was shown. However, it is useful to read others sources in order to understand the language better. More complex objects and commands exist (dictionaries, I/O commands, …) and can be used to improve the efficiency and clarity of an input file.
After reading this introduction to Python, this language can be used to define a first input file in Metafor: