---
fid: 20251025-132058
title: Dict Tutorial 2
tags: dict, dict basic
---

# Dict Tutorial 2

(date: 2025-10-25)

**难度**: 1

**时长**: 30 min



## 获承成员值 dict.get() 

方括号法写起来直观：

```
dict[key]
```

但不能很好的解决 key 不存的的问题。如果要避免 key not in dict error，使用 `get(key)` :

```python title:""
dict.get('key', default_value)
```

Example 1:

```python
students['unknown']
students.get('unknown', '未知')
```


Example 2:

注意下面的陷阱

```python
data = {'name': 'Alice', 'age': None}

# 键存在但值为 None
print(data.get('age', '未知'))      # 输出: None

# 键不存在
print(data.get('gender', '未知'))   # 输出: '未知'
```

详见： [Python dict.get(default) 陷阱：None 值不会触发默认值](dict-get-none-pitfall.md)

## loop with assignment

```python title:""
# Define an empty dictionary
my_dict = {}
 
# Define a list of keys and a list of values
keys = ['a', 'b', 'c']
values = [1, 2, 3]
 
# Loop through the keys and values
for i in range(len(keys)):
    # Add the key-value pair to the dictionary
    my_dict[keys[i]] = values[i]
 
# Print the resulting dictionary
print(my_dict)
```


## dict.setdefault() 
`dict.setdefault()`  to Add keys to dictionary if not exists 

#why/solved dict 为什么要用 setdefault(), 直接等号赋值为什么不行？
set default works only if the key doesn’t already exist in the dictionary. If the key already exists, the `setdefault()` method returns the current value for that key without changing it. 不修改已有值

```python title:""
# Define a dictionary with some initial keys
my_dict = {'a': 1, 'b': 2}
 
# Define a list of keys and a list of values
keys = ['a', 'b', 'c']
values = [1, 2, 3]
 
# Loop through the keys and values
for i in range(len(keys)):
    # Add the key-value pair to the dictionary if the key doesn't already exist
    my_dict.setdefault(keys[i], values[i])
	
	# my_dict(keys[i]) = values[i]   #/// 这样写为什么不行？

 
# Print the resulting dictionary
print(my_dict)
```

## Add tuple as key to dictionary
tuple is hashable for dict key

```python title:""
# Define a dictionary with a tuple key and a value
my_dict = {('a', 1): 'apple'}
 
# Access the value with a tuple key
print(my_dict[('a', 1)])
 
# Add a new key-value pair with a tuple key
my_dict[('b', 2)] = 'banana'
 
# Print the updated dictionary
print(my_dict)
```


## dict comprehension
#toreview
```python title:""
# Define the original dictionary
my_dict = {'a': 1, 'b': 2, 'c': 3}
 
# Define a list of keys to add
keys_to_add = ['d', 'e', 'f']
 
# Define a value for the new keys
value = 0
 
# Create a new dictionary with the key-value pairs to add
new_dict = {key: value for key in keys_to_add}
 
# Use dictionary comprehension to add the new keys to the original dictionary
my_dict = {**my_dict, **new_dict}
 
# Print the updated dictionary
print(my_dict)
```


## Dict方法总结

-   d.clear()
-   d.get(<key>[, <default>])
-   d.items()
-   d.keys()
-   d.values()
-   d.pop(<key>[, <default>])
-   d.popitem()
-   d.update(<obj>)

## References

- [Dictionaries in Python @ Real Python](https://realpython.com/python-dicts/)
- [Effortlessly Add Keys to Python Dictionaries： A Complete Guide @ pythonpool.com](https://www.pythonpool.com/add-keys-to-dictionary-python/)