python︱写markdown一样写网页，代码快速生成web工具：streamlit 缓存（五）

悟乙己

发布于 2021-12-07 14:17:12

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发布于 2021-12-07 14:17:12

文章被收录于专栏：素质云笔记

系列参考： python︱写markdown一样写网页，代码快速生成web工具：streamlit介绍（一） python︱写markdown一样写网页，代码快速生成web工具：streamlit 重要组件介绍（二） python︱写markdown一样写网页，代码快速生成web工具：streamlit 展示组件（三） python︱写markdown一样写网页，代码快速生成web工具：streamlit lay-out布局（四） python︱写markdown一样写网页，代码快速生成web工具：streamlit 缓存（五） python︱写markdown一样写网页，代码快速生成web工具：streamlit 数据探索案例（六） streamlit + opencv/YOLOv3 快速构建自己的图像目标检测demo网页（七）

github代码链接：mattzheng/streamlit_demo

文章目录

整个文档可参考:https://docs.streamlit.io/en/stable/caching.html

当您用@st标记一个函数时。缓存装饰器，它告诉Streamlit无论何时调用函数都需要检查以下几件事:

The input parameters that you called the function with
The value of any external variable used in the function
The body of the function
The body of any function used inside the cached function

cache在后台操作的步骤为：

For example, when the function expensive_computation(a, b), decorated with @st.cache, is executed with a=2 and b=21, Streamlit does the following:

1 Computes the cache key
    
2 If the key is found in the cache, then:

- Extracts the previously-cached (output, output_hash) tuple.

- Performs an Output Mutation Check, where a fresh hash of the output is computed and compared to the stored output_hash.

    - If the two hashes are different, shows a Cached Object Mutated warning. (Note: Setting allow_output_mutation=True disables this step).

3 If the input key is not found in the cache, then:

- Executes the cached function (i.e. output = expensive_computation(2, 21)).

- Calculates the output_hash from the function’s output.

- Stores key → (output, output_hash) in the cache.

4 Returns the output.

1 不适用cache的方式

比如求指数，如果不缓存，重新刷新一次还是需要重新计算

import streamlit as st
import time

def expensive_computation(a, b):
    time.sleep(2)  # 👈 This makes the function take 2s to run
    return a * b

a = 2
b = 21
res = expensive_computation(a, b)

st.write("Result:", res)

2 cache

cache能不能被使用得到，可以通过st.write检测

suppress_st_warning可以关闭一些因为缓存还没计算出，带来的报错

cache状态下,支持随时修改函数的参数
cache状态下,支持任意函数结构改变

@st.cache(suppress_st_warning=True)  # 👈 Changed this
def expensive_computation(a, b):
    # 👇 Added this
    st.write("Cache miss: expensive_computation(", a, ",", b, ") ran")
    time.sleep(2)  # This makes the function take 2s to run
    return a * b

a = 2
b = 21
res = expensive_computation(a, b)

st.write("Result:", res)

3 cache + 可选项

设置一个slider选项，这种情况会发生：

如果之前没有看到过slider里面的数字，那么就会重新执行
如果有，那么就会直接跳出
另外一种，如果接下来这个web会被多人使用，其他人调用过的，也是没有问题的！

功能强大，就不用自己写cache了,简直神器！

@st.cache(suppress_st_warning=True)
def expensive_computation(a, b):
    st.write("Cache miss: expensive_computation(", a, ",", b, ") ran")
    time.sleep(2)  # This makes the function take 2s to run
    return a * b

a = 2
b = st.slider("Pick a number", 0, 10)  # 👈 Changed this
res = expensive_computation(a, b)

st.write("Result:", res)

4 cache 返回字典型

如果返回字典性，会以json的格式展示，非常方便

同时缓存机制有一个比较大的问题: - 比如第一次正常执行res['output'] = 1 - 然而，不改变a/b的情况下，再刷新页面一次，因为a/b没变，所以就没有重新计算expensive_computation(a, b) 就会造成res['output'] = "result was manually mutated"，这个时候就有问题，会报错提醒：

CachedObjectMutationWarning: Return value of expensive_computation() was mutated between runs.

所以尽量避免生成值之后的再赋值。

@st.cache(suppress_st_warning=True)
def expensive_computation(a, b):
    st.write("Cache miss: expensive_computation(", a, ",", b, ") ran")
    time.sleep(2)  # This makes the function take 2s to run
    return {"output": a * b}  # 👈 Mutable object

a = 2
b = 21
res = expensive_computation(a, b)

st.write("Result:", res)

res["output"] = "result was manually mutated"  # 👈 Mutated cached value

st.write("Mutated result:", res)