Use MATLAB Dictionaries in Python

You can convert a Python® dict to a MATLAB® dictionary, and a MATLAB dictionary to a Python dict. In Python code, you can use dict methods on MATLAB dictionaries.

  • You can convert a Python dict to a MATLAB dictionary by first passing it into the matlab.dictionary constructor, and then passing the generated matlab.dictionary object to MATLAB. Alternatively, you can convert a Python dictionary to a MATLAB structure by passing the Python dictionary directly to MATLAB.

  • You can pass a MATLAB dictionary to Python as a matlab.dictionary object. Alternatively, you can create a matlab.dictionary object directly in Python. This object behaves like a Python dict.

Convert Python Dictionary to MATLAB Dictionary or Structure

You can convert a Python dict to a MATLAB dictionary by passing it to the Python matlab.dictionary function and then passing the resulting matlab.dictionary object to MATLAB. (since R2024b)

For example, create a matlab.dictionary object from a Python dict in Python. Then pass the matlab.dictionary object to MATLAB using workspace. In this case, MATLAB converts the keys to strings and the values to doubles.

import matlab.engine
eng = matlab.engine.start_matlab('-desktop')
 
pd = {'milk': 3.50, 'bread': 2.50, 'eggs': 2.75}
md = matlab.dictionary(pd)

eng.workspace['md'] = md

In MATLAB, display the dictionary. 

md
md =

  dictionary (string ⟼ double) with 3 entries:

    "milk"  ⟼ 3.5000
    "bread" ⟼ 2.5000
    "eggs"  ⟼ 2.7500

MATLAB converts the Python keys and values to the equivalent type in MATLAB when possible. If the Python dictionary keys are not all of the same data type after being automatically converted to a MATLAB data type, MATLAB wraps the keys in cell arrays. The same is true of the Python dictionary values.

Alternatively, you can convert a Python dict to a MATLAB structure by passing the Python dict directly to MATLAB using workspace. This conversion requires the Python dictionary keys to be valid structure field names, as described in struct.

 eng.workspace['pd'] = pd

In MATLAB, display the structure. 

pd
pd = 

  struct with fields:

     milk: 3.5000
    bread: 2.5000
     eggs: 2.7500

Python object types other than dict that support the mapping protocol can also be converted to a MATLAB dictionary. Pass these objects directly to MATLAB using workspace. You do not need to convert such an object to a matlab.dictionary object first. For example, convert a Python OrderedDict to a MATLAB dictionary.

import collections
import matlab.engine
eng = matlab.engine.start_matlab('-desktop')
 
od = collections.OrderedDict({'soup': 3.57, 'bread': 2.29, 'bacon': 3.91, 'salad': 5.00})
eng.workspace['od'] = od

MATLAB converts the Python OrderedDict to a MATLAB dictionary. In MATLAB, display the dictionary. 

od
od =

  dictionary (string ⟼ double) with 4 entries:

    "soup"  ⟼ 3.5700
    "bread" ⟼ 2.2900
    "bacon" ⟼ 3.9100
    "salad" ⟼ 5

Convert MATLAB Dictionary to Python Dictionary

When you pass a MATLAB dictionary to Python, MATLAB passes the dictionary as a matlab.dictionary object. The keys and values of the Python dictionary are determined by the default conversion rules for MATLAB types, which are described at Pass Data Between MATLAB and Python. You can then explicitly convert the matlab.dictionary object to a Python dict by using the dict constructor.

For example, create a MATLAB dictionary and convert it to a Python dict.

First, start the MATLAB engine in Python.

import matlab.engine
eng = matlab.engine.start_matlab('-desktop')

Create a MATLAB dictionary in MATLAB.

dm = dictionary(["Avg Temp","Dew Point","Precipitation"],[71.12 69.07 0.0]);

Pass the MATLAB dictionary to Python. Convert the created matlab.dictionary object into a Python dict.

dm = eng.workspace['dm']
dp = dict(dm)

In Python, display the dict. 

dp
{'Avg Temp': 71.12, 'Dew Point': 69.07, 'Precipitation': 0.0}

Use Python Dictionary Methods on MATLAB Dictionaries

In Python code, you can use many Python dict methods on matlab.dictionary objects.

For example, print the length of the matlab.dictionary object dm using len.

len(dm)

3

These operations support matlab.dictionary objects, where dm represents the dictionary:

  • list(dm)

  • len(dm)

  • dm[key]

  • dm[key] = value

  • del dm[key]

  • key in dm

  • key not in dm

  • iter(dm)

  • dm.clear()

  • dm.todict()

  • classmethod fromkeys(iterable, value=None)

  • dm.items()

  • dm.keys()

  • dm.values()

  • dm == dm2 (supported only if dm2 is a matlab.dictionaryobject; returns true only if dm2 contains the same key-value pairs)

The items(), keys(), and values() methods of a matlab.dictionary object return a list. This behavior differs from Python dict, whose corresponding methods return view objects.

See Also

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