breakpoint(): Python’s version of MATLAB’s keyboard() command
callable(): Like MATLAB’s isfunction() but it really checks if there’s a __call__ method
getattr()/hasattr(): MATLAB’s getfield()/isfield(). The 3rd parameter of getfield() is a shortcut to spit out a default if there’s no such field/attribute, which MATLAB doesn’t have
globals()/locals(): more convenient than MATLAB because the whole workspace (current variables) are accessed as a dictionary in Python by calling locals() and globals()
id(): memory address of the item where the variable (reference) is pointing to. Think of it as &x in C.
isinstance(): MATLAB’s isa()
next(): Python favors not actually computing the values until needed so instead it offers a generator (forward iterable) function that spits out one value at each time you kick it with next() and you can’t go back.
chr()/ord(): analogous to MATLAB’s char()/double() cast for characters
Python’s exponentiation is **, not ^ like most other languages (C does not have exponentiation symbol, and ^ was used for xor)
print(…, flush=false) allows a courtesy flush
repr(): MATLAB’s version of disp(), also overloadable standard interface
slice(): MATLAB’s equivalent of colon() special interface
Context Manager
@contextlib.contextmanager decorators basically splits a set-try-yield-finally boilerplate function into 3 parts: __enter__ (everything above yield), BODY (where the yield goes to) and __exit__ (everything below yield), since a with-as statement is a rigidly defined try-finally block, roughly like this:
with EXPR as f:
BODY(using f)
__enter__: f=EVAL(EXPR)
try:
# f isn't evaluated till yield
yield f # Goes to BODY
finally:
__exit__: cleanup(f)
Python uses garbage collectors, so onCleanup() might often work, but it’s not guaranteed to. So any code based on that should not be in production
Answer: Context Manager, a glorified try-catch (more specifically try-finally) block with a rigid structure. It’s a pain in the butt and not fun to deal with if you want to deviate from the native ContextManager that came with the resource opener
‘switch’-case is back as ‘match’-case (the advanced uses are different)
New Python finally supports it, ‘switch’ in C is called ‘match‘ in Python and there are many handy and intuitive syntax just like in MATLAB! Horray!
If you try to do anything fancy with mutables in the cases, be careful about the side effects!
Pass by Variable (Copy-on-Write) like MATLAB
Variables are by large references in Python. Everything including integers are some sort of classes (which are in turn dictionaries with special treatment to certain key names). The garbage collector scans for the last guy using that part of memory not referencing it anymore before cleaning it.
Python even have one ‘None’ for the entire universe with a gazillion things going on pointing to the same memory address where None is stored (that’s why None is idoimatically checked by ‘is’ keyword which checks the address for speed instead of ‘==’ which actually verify the contents for speed). If you look up the reference count (see garbage collector) for commonly used numbers like 1s and 0s, there are thousands of ‘users’ of it!
With C++, it’s a mixture. Complex objects are usually passed by reference for performance reasons but simple structs and data types can be passed as variables (C/C++’s nomenclature calls the non-reference/pointers variables though technically references/pointers are just the same integers identified as addresses) that gets cloned and destroyed when they move across function (stack) boundaries.
In MATLAB, they want it industrial strength, so that’d rather not allow anything insidious/non-transparent to happen in your code by keeping it all pass-by-variable, that is everything is supposed to be treated as different copies as it crosses function boundaries. For performance reason, they figured if you passed a big matrix just for the function to read, MATLAB doesn’t really have to clone that so under the hood you can peak the same matrix that belong to the caller. Once your function changes the contents (they are pretending that it’s a separate copy so of course you can), MATLAB painfully makes a whole copy of it (copy-on-write) which you then have to lug the 2nd (modified) copy around when it travels past function boundaries.
Python takes this idea a lot further by having anything that’s exactly the same (including None or string literals) to point to the same object until you ask to change the contents, then it makes a new copy for you to change and point to the new copy specifically for the variable name you are referencing with.
Answer: The way Python prevents variables passed as parameters passed into a function from getting modified is to separate variables into mutable (lists, dicts, sets), and immutable (tuples, frozendict is a package right now, frozenset, numerics, strings) types. Anything immutable going past the function boundary gets their own local copy.
Classes Boundaries
MATLAB and C++ has stringent access control, but not in Python. There’s not even const correctness. Just signal your intention with variable naming schemes like all caps and __ prefixes.
C++ do not separate helper functions and class (non-instance) methods. What C++ called static members are really just glorified free functions and global variables tucked under the namespace that happened to be in the class (classes started off as namespace for structs then people add features like overloading and dispatch mechanisms). partitioning the global workspace. Whether a scoped helper function call a scoped variable that happened in the same namespace is nothing special to C++
Python does separate these two concepts though. Class method in Python (decorated by @classmethod), on the other hand, are equivalent to C++’s static methods which they are not allowed to touch anything instance-specific, but they can access anything class-specific. Helper functions, which is called ‘Static Method‘ (decorated by @staticmethod) in Python cannot even touch anything specific to the class.
Variable arguments
C++ doesn’t generally do variable arguments because it defeats polymorphism that uses a function signatures (which is a list of your argument types) to figure out which function to dispatch.
MATLAB uses cell to pack variable arguments. The common idioms are varargin{:} and [varargout{1:nargout}]. To accommodate variable arguments, MATLAB have to give up polymorphism but they still have a little bit of it left: they do dispatch based on the first argument type and it’s very useful in avoiding a lot of stupid switching by detecting data types: just use a consistent function name interface and have each data type implement its own method with the same name.
In Python, there’s no such thing as multiple outputs (return variables) on Python: you output a list and it always gets unpacked (just like MATLAB’s deal() function) when you type a list out on left hand side. If the left hand side is a singleton, it will get the full list that’s still packed. If you write out the elements (which makes the left hand side a list), the returned list will have elements assigned to the left hand side depending on your syntax.
This is often a point of agony deciding on output format when I develop MATLAB code. Apparently TMW wondered the same thing too because their own factory code is all over the place on this too. Most of the time it’s not a good idea to have a context-dependent (depending on how many outputs the user supplied) even if you can technically do that by detecting nargout in MATLAB.
Answer: My recommendation is to make sure the simple, most common case got priority, and stuff the juicy side info in packaged data structures (such as array or cells/monads/lists) and stick to a fixed output format whether you are in Python or MATLAB
Late Binding in Lambda / Anonymous Functions: Capture it!
This often throw people off in Python. MATLAB uses early binding, which means when you created that anonymous function (aka lambda), the free variables (parameters that are not running, aka the input arguments to the lambda/anonymous-function) captured the snapshot of the local workspace at the moment the lambda/anonymous-function was created!
Python on the other hand, uses the same approach as C++: late binding. This means you have to explicitly capture (make a snapshot copy) the free variables if you want to associate it with the values when the anonymous function (lambda) was created, not to wait until the lambda was actually called/used to look for what values to use in the free variables.
P = 612
# P was not captured, thus late binding
f = lambda x : print{f'input/running:{x}, param/free:{P}')
# P was captured as p, thus early binding
g = lambda x, p=P: print{f'input/running:{x}, param/free:{p}')
P = 721
f(8964) # shows "input/running: 8964, param/free: 721"
g(8964) # shows "input/running: 8964, param/free: 612"
This article is not for Pythonisms (like ContextManager), etc, the way things should normally be done in Python, but the more non-obvious way to solve problems or new features that are available specifically in Python.
Use / and * argument as separator for different forms of parameter entries!
There is special syntax to separate positional-only, positional/keyword, and keyword-only parameters.
Make a variant of existing class/object
This ninja technique is useful when you want to keep the object mostly the way it is but add/override a few things you think it didn’t do right without inheriting or use composition (hide a copy of object as a member) and write a proxy mirror for every member of it.
In C++, this situation is often used when you want to modify a concrete class that doesn’t have a virtual destructor, most notoriously STL which you are not supposed to inherit from (or else the client might pass a pointer to the parent/base so the child object’s destructors are not called as there are no vtables to keep track of which method to dispatch). In C++, this is often the few use cases that calls for private inheritance.
In Python, because everything is a recursive dict that specially named (magic) functions are recognized, there is a __getattr__ method that gets called whenever a member is accessed, which is the case when the member is called (in Python you simply get the functor as an attribute, aka value in the key-value pairs, in the dictionary and add brackets to call it). This means you can re-route what attributes (members) are returned simply by overloading this method!
If you can overload __getattr__, it also mean you can redefine the member interface of your entire class! So a strategy to make a class have the same exact interface as another class is to hide a copy/reference to the underlying class and re-route the __getattr__ to the underlying object’s __getattr__! Here’s the gist of it:
This can be improved a little bit. Just pick a member name that won’t clash with the underlying object’s attributes/members and simply return the ‘hidden’ object when specifically requested, not self.__obj.__obj.
This is a very powerful luxury that makes Python so lovable if you are not here for industrial strength programming. MATLAB’s classes are hard-wired to your class definition .m file, not something you can update on the fly as you please. Any attempts to do so (aka breaking the safeguards) are Undocumented MATLAB territory where you mess with the Java under the hood and change the metadata property to fool the objects to do what you want.
Python’s import structure is freaking confusing. Learning by examples (i.e. imitating example code) does not help understanding the logic of it, and there are a lot of possible invalid combinations that are dead ends. You need to understand the concepts below to use it confidently!
Just like C++ quirks, very often there’s valid reasoning behind this confusing Python design choice and it’s not immediately obvious. Each language cater certain set of use cases at the expense of making other scenarios miserable. That’s why there’s no best universal language for all projects. Know the trade-offs of the languages so you can pick the right tool for the job.
MATLAB’s one file per function/script design
MATLAB made the choice of having one file describe one exposed object/function/class/script so it maps directly into the mental model of file systems. This is good for both user’s sanity and have behavioral advantages for MATLAB’s interpreter
Users can reason the same same way as they do with files, which is less mental gymnastics
Users can keep track of what’s available to them simply by browsing the directory tree and filenames because file names are function names, which should be sensibly chosen.
Just like users, MATLAB also leverage the file system for indexing available functions and defer loading the contents to the memory until it’s called at runtime, which means changes are reflected automatically.
Package/modules namespace models in MATLAB vs Python
MATLAB traditionally dumps all free functions (.m files) available in its search paths into the root workspace. Users are responsible for not picking colliding names. Classes, namespaces and packages are after-thoughts in MATLAB while the OOP dogma is the central theme of Python, so obviously such practices are frowned upon.
RANT: OOP is basically a worldview formed by adding artificial man-made constructs (meanings such as agents, hierarchy, relationships) to the idea of bundling code (programs) and data (variables) in isolated packages controlled (scoped) by namespaces (which is just the lexer in your compiler enforcing man-made rules). The idea of code and data being the same thing came from Von Neumann Architecture: your hard drive or RAM doesn’t care what the bits stands for; it’s up to your processor and OS to exercise self-restraint. People are often tempted to follow rules too rigidly or not to take them seriously when what really matters is understanding where the rules came from, why they are useful in certain contexts and where they do not apply.
Packages namespaces are pretty much the skeleton of classes so the structure and syntax is the same for both. From my memory, it was at around 2015 that MATLAB started actively encouraging users (and their own internal development) to move away from the flat root workspace model and use packages to tuck away function names that are not immediately relevant to their interests and summon them through import syntax as needed. This practice is mandatory (enforced) in Python!
However are a few subtle differences between the two in terms of the package/module systems:
MATLAB does not have from statement because import do not have the option to expose the (nested tree of) package name to the workspace. It always dumps the leaf-node to the current workspace, the same way as from ... import syntax is used in Python.
MATLAB does not have an optional as statement for you to give an alternative name to the package you just imported. In my opinion, Python has to provide the as statement as an option to shorten package/module names because it was too aggressively tucking away commonly used packages (such as numpy) that forcing people to spell the informative names in full is going to be an outcry.
Unlike free functions (.m files), MATLAB classes are cached once the object is instantiated until clear classes or the like that gets rid of all instances in the workspace. Python’s module has the same behavior, which you need to unload with del (which is like MATLAB’s clear).
Python’s modules are not classes, though most of the time they behave like MATLAB’s static classes. Because the lack of instantiated instances, you can reload Python modules with importlib.reload(). On the other hand, since MATLAB packages merely manages when the .m files can get into the current scope (with import command), the file system still indexes the available function list. Changes in .m file functions reflects immediately on the next call in MATLAB, yet Python has to reload the module to update the function names index because the only way to look at what functions are available is revisiting the contents of an updated .py file!
MATLAB abstracts folder names (that starts with + symbol) as packages and functions as .m files while Python abstracts the .py file as a module (like MATLAB’s package) and the objects are the contents inside it. Therefore Python packages is analogous to the outer level of a double-packed (nested) MATLAB package. I’ll explain this in detail in the next sections.
Files AND directories are treated the same way in module hierarchy!
This comes with a few implications
if you name your project /myproj/myproj.py with a function def myproj(), which is a very usual thing most MATLAB users would do, your module is called myproj.myproj and if you just import myproj, you will call your function as myproj.myproj.myproj()!
you can confuse Python module loader if you have a subfolder named the same as a .py file at the same level. The subfolder will prevail and the .py file with the same name is shadowed!
The reason is that Python allows users to mix scripts, functions, classes in the same file and they classes or functions do not need to match the filenames in order for Python to find it, therefore the filename itself serves as the label for the collection (module) of functions, classes and other (script) objects inside! The directory is a collection of these files which itself is a collection, so it’s a two level nest because a directory containing a .py file is a collection of collection!
On the other hand, in MATLAB, it’s one .m file per (publicly exposed) function, classes or scripts, so the system registers and calls them by the filename, not really by how you named it inside. If you have a typo in your function name that doesn’t match your filename, your filename will prevail if there’s only one function there. Helper functions not matching the filename will not be exposed and it will have a static/file-local scope.
Packages in MATLAB are done in folders that starts with a + symbol. Packages by default are not exposed to global namespaces in your MATLAB’s paths. They work like Python’s module so you also get them into your current workspace with import. This means it’s not possible to define a module in a file like Python. Each filename exclusively represent one accessible function or classes in the package (no script variables though).
So in other words, there are no such thing called modules in MATLAB because the concept is called package. Python separated the two concepts because .py file allowing a mixture of scripts, classes and loose functions formed a logical unit with the same structure as packages itself, so they need another name called module to separate folder-based collection (logical unit) and file-based collections (logical unit).
This is very counterintuitive at the surface (because it defeats the point of directories) if you don’t know Python allowing user to mix scripts, functions and classes in a file meant the file itself is a module/collection of executable contents.
from (package/module) import (package/module or objectS) <as (namespace)>
This syntax is super confusing, especially before we understand that
packages has to be folders (folder form of modules)
modules can be .py files as well as packages
packages/modules are technically objects
The hierarchy for the from import as syntax looks like this:
package_folder > file.py > (obj1, obj2, ... )
This has the following implications:
from strips the specified namespace so importdumps the node contents to root workspace
import without fromexposes the entire hierarchy to the root workspace.
functions, classes and variables in the scripts are ALL OBJECTS.
if you do import mymodule, a function f in mymodule.py can only be accessed through mymodule.f(), if you want to just call f() at the workspace, do from mymodule import f
These properties also shapes the rules for where wildcards are used in the statement:
from cannot have wildcards because they are either a folder (package) or a file (module)
import is the only place that can have wildcards * because it is only possible to load multiple objects from one .py file.
import * cannot be used without from statement because you need to at some point load a .py file
it’s a dead end to do from package import * beacuse it’s trying to load the files to the root workspace which they are uncallalble.
it also does not make sense (nor possible) to follow import * with as statement because there is no mechanism to map multiple objects into one object name
So the bottom line is that your from import as statement has to somehow load a .py file in order to be valid. You can only choose between these two usage:
load the .py file with from statement and pick the objects at import, or
skip the from statement and import the .py file, not getting to choose the objects inside it.
as statement can only work if you have only one item specified in import, whether it’s the .py file or the objects inside it. Also, if you understand the rationales above, you’ll see that these two are equivalent:
from package_A import module_file_B as namespace_C
import package_A.module_file_B as namespace_C
because with as statement, whatever node you have selected is accessed through the output namespace you have specified, so whether you choose to strip the path name structure in the extracted output (i.e. use from statement) is irrelevant since you are not using the package and module names in the root namespace anymore.
The behavior of from import as is very similar to the choices you have to make extracting a zip file with nested folder structures, except that you have to make a mental substitution that a .py file is analogous to a subfolder while the objects described in the .py file is analogous to files in the said subfolder. Aargh!
* Shallow assignment (transferring reference only) means the LHS does not have its own copy, so modifying the new reference will modify the underlying data on the RHS.
| Format-List -Property * or Format-List -InputObject
properties() methods() get()
List members (method and properties)’s prototypes
| Get-Member
Powershell specific
The UNCAPTURED output value in the last line of the block is the return value! Unary side effect statements such as $x++ do not have output value. Watch out for statements that looks like it’s going nowhere at the end of the code as these are not nop/bugs, but return value. This has the same stench as fall-throughs.
foreach() follows the last uncaptured output value return rule above doing a 1-to-1 map from the input collection to output collection (you can assign output to foreach() as it’s also seen as a function)
Powershell suck at binary operations between two arrays. Just an elementwise A+B you’d be thinking in terms of loops and worry about dimensions.
You can put if and loop blocks inside collections list construction, like this:
When used with classes and custom matrices/arrays, chaining fields/properties/methods by indices often do not work, when they do, they often give out only the first element instead of the entire array (IIRC, there are operator methods that needs to be coordinated in the classes involved to make sure they chain correctly). In short, just don’t chain unless in very simple, scalar cases. Always output it to a variable a access the leaf.
Negative (cyclic) indexing along with automatic descending range, along with the lack of ‘end’ keyword is a huge pain in the rear when you want to scan from left to right like A[5:end].
Instead, you’ll have to do $A[4..($A.length-1)] because the range 4..-1 inside A[4..-1] is unrolled as 4,3,2,1,0,-1 (thus scanning from right to left and wraps around) without first consulting with the array A like the end keyword in MATLAB does so it can substitute the ends of the range with the array information before it unrolls.
I am willing to bet that this behavior does not have a sound basis other than people thinking negative indices and descending ranges alone are two good ideas without realizing that nearly nobody freaking wants to scan from right to left and wrap around!
I had the same gripes about negative indices in Python not carefully coordinating with other combinations in common use cases which cases unintuitive behavior.
Range indexing syntax
# Powershell
1..10 # No step/skip for range creation
A[1..10] # No special treatment in array such as figuring out the 'end'
% MATLAB
A[start:(step):stop]
# Python
A[range(start,stop,step)]
# Slicing (it's not range)
A[(start):(stop):(step)] # Can skip everything
# In Python, A=X merely reassign the label A as the alias for X.
# Modifying the reassigned A through A=X will modify underlying contents of X
# To deep-copy contents without .Clone(), assign the full slice
A[:] = X
Hasthtable / Dictionaries
% MATLAB: Use dynamic fields in struct or containers.Map()
# Python: dictionaries such as {a:1, b='x'}
# Powershell: @{a=1, b='x'}
Structs
Powershell does not have direct struct or dynamic field name struct. Instead if your object is uniform (you expect the fields not to change much), use [PSCustomObject]@{}. You can also just use simple hashtable @{}, but for some reason it doesn’t work the way I expected when put into arrays when I try to reference it by array index.
Array rules surprises
Array comparisons are filtering operation (not boolean array output like MATLAB). (0..9) -ge 5 gives 5 to 9, not a list of False … False, True … True. To get a boolean array, use this shortcut:
(0..9) | % {$_ -ge 5}
Map-filter combo syntax is | ? instead of Map syntax | %
Monad (Cells in MATLAB) are unpacked and stacked by default (in MATLAB, I had to write a lot of routines to unpack and stack cells of cells). To keep cells packed (in MATLAB lingo, it’s like ‘UniformOutput’, false in cellfun), add a comma unary operator in front of the operation that are expected to be unpacked like this:
.$_.Split('_')
Set Operations
This is one of the WTF moments of Powershell as a programming language. Convenient set operations is essential for most of the routine boring stuff that involves relational data. A lot of Powershell’s intended audience works in database like environment (like IT managers dealing with Active Directory), they have Group-Object for typical data analysis tasks, yet they make life miserable just to do basic set operations like intersection and differencing!
Powershell has a Compare-Object, but this is as unnatural and annoying to use as users are effectively rebuilding all 4 basic set-ops (intersection, union, set-diff, xor) based on any two! Not to mention you have to sift through table to get to the piece you wanted!
Basically Compare-Object out of the box
is a set-diff showing both directions (A\B and also B\A) at the same time. If you throw away the direction info, it’s xor.
if you want intersection, you’ll need to add -IncludeEqual -ExcludeDifferent
(WTF!) If you just specify -ExcludeDifferent, by definition there’s no output because by default Compare-Object shows you ONLY the two set-diffs and you are telling it to not show any diffs!
Union is specifying -IncludeEqual only. But it’d rather stack both then do a | Sort-Object - Unique
Some people might suggest doing | ? {$_ -eq $B} for intersection (or is-member). This is generally a bad idea if you have a lot of data because it’s in the O(n*n) runtime algorithm (loop-within-loop) while any properly done intersection algorithm will just sort then scan the adjacent item to check for duplicates, which gives O(n log(n)) time (typical sorting algorithm takes up most of the time).
If you noticed, it’s set operations within the outputs of Compare-Objects with the Venn diagram of -IncludeEqual -ExcludeDifferent switches! It’s doable, but totally unnecessary mindfuck that should not be repeated frequently.
In MATLAB land, I made my own overloading operators that do set operation over cellstr(), categorical and tabular objects (I went into their code and added the features and talked to TMW so they added the features later), sometimes getting into their sort and indexing logic as necessary. This shows how badly do I need set operations to come naturally.
One might not deal with it too much in low level languages like C++ (STL set doesn’t get used as much compared to other containers), but for a language made to get a lot of common things done (i.e. the language designer kind of reads the users mind), I’m surprised that the Powershell team overlooked the set operations!
Sets are very powerful abstractions that should not be made less descriptive (hard to read) by dancing around it with equivalent operations with some programming gymnastics! If these basic stuff are not built in, we are going to see a lot of people taking ugly shortcuts to avoid coding up these bread and butter functions and put it in libraries (or downloading 3rd-party libraries)!
Powershell surprises
Typical symbolic comparison operators do not work because ‘>’ can be misinterpreted as redirection in command prompts. Use switches like -gt (greater than) instead.
Redirection’s default text output uses UTF16-LE encoding (2 bytes per character). Programs assuming ASCII (1 byte per character) might not behave as intended (e.g. if you use copy command merge an ASCII/UTF8 file with UTF16-LE, you might end up with spaces in the sections that are formatted with UTF16-LE)
Cannot extract string matches from regex without executing a -match which returns boolean unless we use the the $matches$ spilled into variable space. Consider [regex]::Match($Text, $Pattern).Groups[1].Value
Methods are called with parenthesis yet functions are not called with parenthesis, just like cmd-lets! Trying to call a function with multiple input arguments with parenthesis like f(3,5) will be interpreted as calling f with ONE ARGUMENT containing an ARRAY of 3 and 5!
Write-Host takes everything after it literally (white spaces included, almost like echo command), with the exception of plugging in $variables! If you want anything interpreted, such as concatenation, you need to put the bracket around the whole statement!
Libraries and Modules
Reload module using Import-Module $moduleName -Force
